While reading an article about possible expanded service of Amtrak’s Vermonter, I discovered a new prototype I had never seen, the DMU. These are interesting looking units, modern day RDC’s I guess you could call them. While I don’t model modern day New England, my club does, it would be cool to have a model of these running the Vermonter if Amtrak decides to go that way.
The term DMU = Diesel Multiple Unit and the concept of powered coaches that can be consited together have been around in Europe for a long time, at least back to the 30s. Their close cousin the EMU = Electric Multiple Unit has been around even longer.
I’ve never quite got my head around why it has never been that popular in the US (except in certain areas like Boston where the Budd RDCs sold in numbers) where it seems loco haul still dominates passenger traffic. I’ve read somewhere that its to do with FRA crash safety rules, but that seems a bit odd, certainly the latset crop of high speed EMUs and DMUs in Europe are extremely crash resistant - as many are travelling regularly at over 200 km/h they have to be.
I’m with boxcar jim on wondering why the Budd RDC was the only mass-produced DMU in US service - and not very much US service, at that.
In Japan the DMU caught on for local and (non-electrified) commuter service. Then, in the 1960’s, the Kokutetsu introduced high speed DMU limited expresses (with chair cars and diners) on some major non-electrified routes. The dedicated control units were apparently reinforced, not unlike the modified RDCs retired from Amtrak service, while the cars intended to make up the main part of each consist had hostler controls only.
The one problem I can see with this concept is limited fuel capacity - the longest run the JNR had for these DMU expresses was about equivalent to Los Angeles - Sacramento. Adding a fuel tender to a DMU train would almost certainly be a non-starter. OTOH, regional local service and commuter use wouldn’t be unduly handicapped by limited range.
Chuck (modeling Central Japan in September, 1964 - with LOTS of DMUs)
One of the big reasons is flexibility.Look at it this way. A six car set, with loco. The loco blows the engine - you go get another and go. Lets say the middle car loses brakes - you already have the loco power to set off the bad car and go.
Now, 6 DMUs. One blows the motor. You have to wait for a crew and loco to get the bad car and set it off somewhere. Then find another car to complete the set, hook it up before you can go. Subways and els are powered fom outside the car, so the failure rate is very low and there is no maintenance for diesel engines. Budd RDCs were very low power and trying to pull another car voided the warranty.
DMU operation, JNR style. One engine goes out on a six car set. The set continues to destination, on schedule, with (depending on class of DMU) nine to eleven operating engines. Upon arrival, the diesel engine is swapped out and replaced by a like serviceable item (designed for quick engine change.)
Single-locomotive operation, US style. The engine dies. The train sits wherever until a relief locomotive can be found and dispatched to the scene. In the meantime, passengers miss getting to work on time, important business meetings, flights - and maybe that one big sale that only happens today or not at all.
Assuming you were a passenger, which would YOU prefer?
Chuck (modeling Central Japan in September, 1964)
Chuck is dead right - DMUs provide more reliability as if one engine or car fails the train has a fighting chance to continue to its destination unaided vs loco haul where you have to wait for a replacement engine. This is certainly the experience I have of travelling on DMUs in the UK during the 80’s and 90’s. I would also guess that on Britain’s crowed rail network its one reason why they proved popular on none electrified routes so delays don’t start building up. Maybe the Budd RDCs were just under powered? We are certainly talking 50’s era diesel engine technology and things have come a long way since then. Maybe its back to the crash resitance thing again and US passenger cars have to have more weight in them?
I do wonder about the range issue, there certainly might be something in that for long haul. The longest DMU trips I’m aware of in the UK are about 400 miles, I don’t know if the sets are are fueled after each trip - I suspect not.
Most of my working rail life DMMUs and DEMUs (Diesel mechanicals and Diesel electrics) have been whizzed around at night to top off the fuel tanks and otherwise service them.
The big thing with any MUs is that they are almost all push-me-pull-yous with driving cabs at both ends and a guards/conductors compartment somewhere in the middle. Older one usually had baggage facility as well. The latest often lack a specific place for crew except the cabs and, in some, a galley for food service (of a kind).
Apart from lack of power the Budds also probably just came too late… they arrived at the same time as epanded air travel and even more autos. they just could not compete/the traffic had already started to go if not gone. Look waht happened to all the short branch lines with one car DMMUs in the UK…
On the other hand both the UK and Europe - and Japan - have used long-range/fast MU arrangements for a long time. The “Blue Pullman” and HSTs were effectively multiple units. So, I think, is the Shinkansen. US practice tends - or tended - to put a loco or locos on one end and wye the train with a service break at each end of the run… but don’t the Cabbages made out of old FP45s break this trend? I don’t know how far or where these run. I assume that control lines are run and jumpered through the trains in the same way that we link locos and cab cars on the “Swallows” (or whatever they were called) in and out of Euston and Kings Cross?St Pancras.
One thing I’ve never understood is why the Budds had the (engine?) blister on the roof and didn’t run bus engines under the floor like the Western and Midland DMMUs??? This Budd arrangement must have created all sorts of problems. One of the things with our DMMUs was that any wonky engine could be dropped out … preferably while it was in for a service…and replaced relatively easily.
[8D]
The problem with the Budd RDCs was with the Allison torque converters that they used. They were the weak link in the drivetrain. That’s why Budd got so upset when railroads tried to pull passenger cars with RDCs.
BTW, the Allison torque converters were similar to automatic transmissions like are in autos, only larger. At the time, automotive automatics weren’t all that reliable either.
The Colorado Railcars DMUs use Allison torque converters as well. However the technology has matured over the last half century and can handle twice the power that the RDC era ones could.
Excess weight and crash resistance weren’t a problem. Actually the very opposite was the problem. RDCs were very light when compared to locomotives. On the NH this caused them to not activate the crossing gate sensors, particularly during the winter, and the NH ended up running them with diesels, which kind of defeats the purpose of running RDCs.
I believe the Candian railroads used their RDCs for several medium distance runs.
[quote user=“Dave-the-Train”]
One thing I’ve never understood is why the Budds had the (engine?) blister on the roof and didn’t run bus engines under the floor like the Western and Midland DMMUs??? This B
I believe the reason put forth against the use of DMU’s in the US, at least, is that each car with an operator’s cab is considered a locomotive for inspection purposes under FRA rules. Each car so equipped has to be inspected every 92 days; while a straight coach (around here they are called “blind” as opposed to a “cab car”) is inspected much less often. I’m sure that some one here can fill us in on what is required in the 92 day inspection, and in the coach inspection.
The extra inspections, and the handling of the equipment to get them into position to do the inspections, is the reason DMU’s are passed over when selecting equipment.
A similar situation is also coming up under wires. The original Metroliners were MU’s while the Acela is 2 power cars (locomotives) and six passenger cars. NJ Transit, SEPTA and MARC have also opted for electric locomotives and conventional coaches under wires, although that may be in part to allow more flexibility in the coach fleet since all of them also have diesel-powered lines.