You are talking about equipment utilization, not line capacity.
Line capacity is a function of minimum headway - which doesn’t really change with speed. If you have a line that can maintain 4 min headways, that’s 15 trains an hour. If you double the speed, you need to double the spacing between trains because braking distances double. Four mile spacing at 60 mph has to become eight mile spacing at 120 mph or it could become two mile spacing at 30 miles an hour. It’s still 4 minute headway = 15 trains an hour.
There are three other tricks that can increase capacity w/o lengthening platforms. Decrease seat pitch, decrease seat width, double deck. If you decrease trip times, you can probably do these things w/o degrading comfort too much.
For example, if NYP-PHL trip times go from 1:10 to 0:40, would you really mind a bit less leg room, if it meant keeping the same fare?
Headway of 5-10 minutes should be plenty. As it is, there are not enough Acelas and NE Regionals at peak times, perhaps because of insuffiecient equipment?
The NEC from Newark to NYP is at capacity now during rush hour with 17(?) trains an hour - including NJT. There was a nice article about NYP recently in Trains that led me to believe that platform capacity was pretty much at the limit, too. More folks per train-foot would be the only short term solution…
But if you redesign your signals and tracks to allow for higher speed, the headways can be shortened. And if you can then send trains with 10 or 12 minute headways, you’ve increased capacity. Allowable speed can come about only by improved track and signals and not just saying “go faster”.
You are correct when you say that increasing train frequency increases capacity. This has nothing to do with speed. Increasing speed does not increase frequency.
The limit is safe (and for passenger trains - comfortable) braking rate - which is simple physics.
If you have a line that is at capacity, with signalling optimized for the current speeds, there is nothing you can do to increase capacity, no matter what you do to the signaling.
You have to have braking distance between trains. Braking rate is constant. Distance increases proportional to speed.
Amtrak needs separate, dedicated tracks, not shared with commuter trains (or freight, for that matter) in the NEC. I suppose the bottleneck is the Hudson tunnel. How many tracks is it? Is that why there was a proposed new tunnel that was rejected by Christie?
But you design the whole system to increase capacity: increased speed is the result of track, signal, and locomotion…all three can be designed for braking distances, headways, size of trains, speeds, etc. You cannot go 100 miles an hour with a bicycle or on poor track, or with a manual flag system. It stands to reason that if you increase speeds you can increase capacity…on that 100 mi stretch at 50 MPH it would take a train two hours to go end to end but only one train can run at a time. So at 75 mph it would take 1 hr 20 minutes and at 100 mph one hour. At 75 MPH, therefore, can run a train every 80 minutes and at 100 mph you can run a train every 60 minutes…you have increased capacity with increased speed.
In a way, both POV’s have merit. Let us assume that point NYP to point DC is 200 miles. In a 4 hour period, we can have trains depart one way every 30 minutes. If they average 50 mph, we can only get one train from point to point. If 100 mph, twice as many. That ratio lowers obviously over a 10-15 hour day, but the equipment utilization advantage of higher speeds also kicks in.
A moving-block signal system can increase capacity over a fixed signal system. Moving block systems without wayside signal and only cab signals and automatic preempting safety train control could do the job and are possible within today’s technology.
One side of this argument is measuring headways in minutes and in that frame of reference it is valid.
If you measure headways in signal blocks or miles you will get a different result. The faster a train clears a block, the sooner the block is available for the next train.
Yes Phoebe-Vet, but the faster the allowable train speed, the longer the block, so the two cancel. The only way to increase capacity is by going to moving blocks, and this comes close to doubling capacity. In other words the distance between signal spacing for the speed becomes, with a moving block system, the safety block behind the moving train. Whereas, with fixed blocks, the train behind sees a red signal, until the train ahead is actually two blocks ahead.
Moveable blocks and cab signals are the keys to speed and capacity…but still commuter roads find getting another train in an hour by speeding things up works.
Of course better and more reliable braking can allow higher speeds while retaining an existing block system. Possibly commuter railroads with captive consists not interchanged should consider magnetic track brakes which allow modern light rail cars to mix with traffic and operate in pedestrian zones with excellent safety. They would be used only in emergency brake applications. West Penn used them on interurban cars built in 1912. Standard on all PCC streetcars, but I do not believe they were retained on the Spam Cans (CTA 6000’s). They were used on the C&LE “Red Devils” and the Indiana “High Speeds.”
The Hudson tunnels (aka North River Tunnels) consist of 2 tubes with one track each. So you have a double track line connecting New York Penn to anything south.
Additional tracks are sorely needed for more capacity. Which was what ARC that Christie killed was supposed to bring in its way. And what Amtrak is exploring adding with its Gateway Program.