In North American we have a large number of signaling aspects, yet the Brits only have 4. I know there are some unique differences between us and them, but come on…if they can get by with only four, why do we need a few dozen?
I do not know why either. Remember that in England they belive in keeping things simple. Over here it is see how complex we can make it.
The Brits do a lot of route signalling. If you come to a junction, you will have a signal for each route available. The driver is expected to know the speed restrictions for the curves at the junction, so all he has to be told is whether there is any obstruction ahead he has to slow down and stop for.
The biggest difference between railroading in the UK and North America is the size of the freight trains. In the UK the moving of freight is secondary to the movement of passenger trains and UK freight trains are pitifully small in comparision to North American standards.
Normal manifest trains in North America routinely operate with 7500 tons or more of train and train lengths of 7000 to 12000 feet. Handling and stopping trains of such weight and lengh is a process that requires giving the engineer all possible advance notice of how he must operate his train at the next signal.
Remember, way back when, traffic signals in the US only had 2 indications…Red=Stop and Green=Go…shortly there after is was determined that there needed to be a Yellow indication between the Green and Red indications in order to permit the intersecion to clear and minimize accidents. The same principles apply to the development of railroad signalling in the US. As needs have demonstrated themselves additional signal indications have been developed in order to handle heavier, longer and faster freight trains.
One of the other differences between the UK and US is that in the UK there’s more of a mix of operating speeds between passenger and freight trains. By and large in the US, most railroads just carry freight.
On the Channel Tunnel Rail Link there are no lineside signals. Instead the driver is provided with a continous display in the cab telling him how fast he can go. This is based on the system the French use on their highspeed lines but for the Channel Tunnel and the CTRL are more sophisticated version was developed as these lines are used by freight as well as a high speed passenger trains. (In the Chunnel itself trains fall into 3 basic categories - Eurostar high speed trains, freight trains and the Shuttle trains which carry cars and trucks).
Although the CTRL is intended primarily for high speed passenger trains, provision has also been made for freight trains to use it, and a number of loops provided to enable freight trains to be overtaken by passenger. The passenger trains will operate at different speeds too; Whereas the Eurostars can run at 185mph on the CTRL, the domestic trains from the Kent Coast towns will run at 140mph.
In France and elsewhere in most of Europe on their classic linesI think they have more than 4 aspects; also like in the US their signal is speed based. (Some years in Britain there were a couple of US style speed based installations, one on the suburban line from London Euston to Watford, the other in W. Yorkshire near Dewsbury but both have now been replaced by more conventional 4 aspect signals).
And now there are steady arrow, flashing arrow, signs - all “aspects” to help “direct” traffic. But the basic system still only has three aspects - Stop, Go, and (depending on your frame of mind) “hurry up before the light turns red” or “slow down, the light is about to turn red…”
In reality, US railroads only have three aspects (4 or 5 if you count flashing). All the various configurations are simply methods of conveying specific information in various situations. F’rinstance, red over green (or red over green over red) is “Limited Clear” (NORAC 281c) - proceed at limited speed until you clear the interlocking, then have at it.
Out on the road, block signals often have/had two heads - home and distant, with the distant head indicating the state of the next signal. Taken together, the two heads on the mast can potentially show something like six conditions, from both blocks clear to both blocks occupied, but the individual heads themselves are still only showing three aspects.
The Brits have only 4 aspects and to many of their drivers can still not stop for a RED.
Err, would you care to elaborate - the UK ‘signal passed at danger’ (SPAD) statistics are pretty good, and getting better - see http://www.rssb.co.uk/pdf/year-end-spad-overview-2005.pdf (there were 339 SPAD’s in 2005 compared with roughly 8 million trains run over the same period).
Do you have any SPAD statistics for other railways/railroads ?
There are actually now 6 aspects used in the UK - red, single-yellow, double-yellow, green, flashing single-yellow, flashing double-yellow. There is also a flashing green aspect used to authorise 140mph running (for test purposes only) on one section of the ECML near Peterborough.
The flashing yellow aspects are used on the approach to medium speed junctions to give advance warning that the divergent route is set, so that the driver can brake appropriately. Also ‘approach control’ is used at many junctions - this holds signals at caution or stop until the system works out that the speed of the train is safe for the route to be taken.
So the current UK signalling is really a hybrid of route and speed signalling, but the various caution (yellow) aspects don’t denote specific speeds, merely that the next signal ahead is at a more restrictive aspect. Approaching a stop (red) signal on plain line, the sequence of aspects would be green, double-yellow, single-yellow, red - and all stop signals are absolute, movements past stop signals are only authorised in special circumstances (like equipment failure or engineering works) or by subsiduary signals for limited switching moves etc.
The lights may be simple, but the wiring is complicated as hell…
Short answer-Efficiency
We could use two aspects, red-stop, and green-procede. Of course each train would have to move at a speed allowing it to be able to stop short of each signal. That would waste a lot of effort and time slowing and speeding up at signals. So we have multiple aspects that indicate to the crew what speed they need to be going well ahead of time so they can safely adjust there speed before that point yet maintain max speed untill the need to reduce / stop. Eventually signaling will get to the point where the “blocks” are moveing and reflect a safe brakeing distance between you and the train ahead, or any fixed speed restrictions (at least on high density lines).
The Chicago Transit Authority has a similar set-up in its cab signaling and speed control system, with wayside signals only on the Congress line, both subways and at junctions. Of course, virtually every train has similar equipment and performance characteristics, which makes designing the system much easier than for a common carrier railroad, where equipment and performance can vary widely.
There is also the historical situation, which I’m sure DutchRailNut is well aware of.
MetroNorth Engineers, for example, have to be conversant with the systems put in place by both the New Haven and New York Central - approach Brewster (oh, I’m sorry, SouthEast) from Wassaic and you’ll understand what I mean.
In Britain there was a standardisation from the early '50’s which made signalling more straightforward, although I’d agree with previous posters that UK signalling depends upon route knowledge. There have been several major accidents - Ladbroke Grove being the most recent - where an engineer/driver/motorman has misunderstood or misread a signal with disastorous consequences.
Currently Metro North only has 3 aspects.
Red (top and bottom)= stop
green flashing (middle light) = proceed by cab signal indication.
Red and green alternatly flashing = Absolute block for cab signal failures)
Are you suggesting that the UK engineered without efficiency as a requirment? [?]
Not true…the three-colour signal was invented first. Some municipalities in the US tried a two color signal to cut costs. However people ran the red light on the excuse that they had no notice of it coming, so eventually yellow was everywhere. The result? >> People still run the red and if you ask an offending driver you will hear all sort of excuses except the real reason…selfishness.
Okay so let that be the ‘bar’ that is set for the yellow, or maybe a flashing yellow, and let a lower requirement be the yellow. Many signaling systems around the world bascially said "lets take our worse case scenario, and make that our advance notice (double yellow / flashing yellow), let our light weight and/or slow speed traffic get a single steady yellow, and be done with it.
Most of ye in hear arugue that simplicity is a requirement for a RR to be cost effective, yet we turn around and argue that we need a signal system with dozens of aspects? I know 3 aspects is better than two, 4 better than 3, 5 better than 4, etc, in terms of theoritical efficiency and keeping trains fluid, but the higher costs assocated (with signaling systems with plenty of aspects (maintence costs, engineering costs, procedural costs, accident costs associated with human-signal ‘confusion’, etc)) I would argue would off-set most if not all the gains made.
Yes? maybe no?
In 4 aspect UK signalling, full braking distance - usually around 0.5 to 0.75 miles - is provided between the double-yellow and red signals (e.g. passenger from 125mph, freight from 75mph), but slower passenger trains with high-performance brakes may well be able to run at speed up to the single yellow and stop in half that distance. On lines with lower traffic levels (but still busy by US standards), three aspect (green/yellow/red) or two aspect (green/red preceeded by green/yellow distant signals) signalling is used where full braking distance is from the yellow to red signals - which are obviously spaced farther apart than with 4 aspect signaling.
As to the simplicity versus efficiency argument - I would agree with your argument. The serious accident at Ladbroke Grove (on the approaches to Paddington station in London) that Simon mentioned was a good (or bad) example of this. The immediate cause was a (relatively inexperienc
I read some of the stuff at the davros.org site and I found this piece of test interesting: http://www.davros.org/rail/signalling/articles/junctions.html
“The techniques described above sufficed until the introduction of the HST (also known as the Inter-City 125). These trains were designed to be able to stop from 125mph in the same distance as any other train could stop from 100mph. The problems occurred when these met approach control from yellow. Imagine an HST proceeding at 100mph on a clear day. The driver sees a double yellow signal. He knows that his brakes are capable of stopping him in well under the full braking distance, particularly with a clean, dry, rail, so he continues at the same speed. The next signal is just out of sight around a curve, and as he enters the curve he starts braking gently in order to stop in two signals time. Suddenly he is faced with a junction indicator and the need to brake hard to 70mph for the turnout. Even if he can brake safely, it is not a nice position to be in and the drivers, not surprisingly, complained. After all, they were driving safely and according to the rules - the driver in the example could have safely stopped before a red signal. It is the overloading of the system with a new meaning that was the problem. The solution was to introduce two new signal aspects for high-speed junctions”
The bolded text is what caught my mind, espe
In the UK it’s basically route (not speed) signalling, so all that a caution aspect means is that the next signal is displaying a more restrictive aspect - it doesn’t mean ‘run at medium speed’ etc. It’s up to the driver to judge the braking so that the train can stop at the red signal, based on the braking performance of the train and rail conditions etc.
The Inter-City 125 passenger trains used in the example have much better acceleration and braking than the older trains which the signal spacings (and braking distances) were designed for. To exploit this performance they have to be driven like sports cars rather than heavy trucks e.g. if a train is running late, a good driver will use the fact that in good conditions it’s possible to (safely) brake later from lower speeds and hence recover lost time. The problem was that until they got close to the signal immediately before a junction - which has a routing indicator - they didn’t know for sure if they were going to take a diverging (speed restricted) route or not. This is why the flashing yellows were added to give a positive advance junction indication - a driver can then make an informed decision about braking.
Tony
also depends on speed of trains, density of traffic, siting and various other factors.
4 aspect signalling occurs on the most densely used lines; basically ot give the most block sections available. Trains although timetabled to achieve constant greens in theory usually end travelling on consistent 2 yellows.
Brewster - check yer facts son. In my profession I have taken calls from drivers who have hummed away at 110Mph and then seen the next signal at red due to a failure of some description. They do not know what is around the corner and you wonder what goes thru their minds. when I have picked up that phone all you can hear in their voice is pure terror.