I cannot believe this is still going on ! :roll:
The definitive answer has been given several times already, but for ease of reference I will repeat it.
When
introduced HSTs were treated as DEMUs. The reason for this was as follows:
1) They were designed to be operated as fixed formation sets ;
2) The power cars were not capable of working any other type of train ;
3) The vehicles could (and cannot) not be operated as part of a standard loco hauled train because they had / have different electrical wiring, and operating voltages, and thus the locomotive could not provide an ETS (Note the letters as opposed to ETH) supply to them;
4) The drawgear was then unique in being a fixed buckeye ;
5) There were (are) no buffers ;
6) The braking system is modified to be a dual two-pipe system and requires both circuits to be in use;
7) HST sets had enhanced braking pressures, which other conventional LHCS (excluding Mk4) does not have ;
8 ) The air suspension was operated from the Main Reservoir pipe ;
9) The vehicles were through-wired for train control ;
10) The whole electrical supply (ETS) for the onboard services was provided from the rear power car, which ran at a slightly faster idling speed, unlike other ETH LHCS, which uses the ETH supply to trickle charge batteries under each vehicle ;
11) None of the vehicles were equipped with lamp brackets and thus they could not convey a tail lamp ;
12) There was no compatible brake van that could be used in operational service
13) For the purposes of signalling rules and regulations, a train was allowed to proceed without a tail light provided that trainset was complete with rear power car.
14) HSTs were only permitted to assist another HST
After initial running experience, as I have already said some pages back, it was established that the fixed formation, whereby the whole trainset was maintained as one, was not working, and the whole 253/254 numbering system was abandoned. At that stage the individual power car numbers were painted on the front, in the series 43xxx.
Unlike locomotives, the HST power cars came under the control of the owning depot and were NOT under the control of the Traction Maintenance & Running Controllers in the Regional Control at York.
Although for Operating purposes, HST power cars are treated as locomotives when running either singly or in pairs, they are not true locomotives because :
a) They do not have conventional buffing gear ;
b) They do not have a PASS/GOODS brake timing changeover switch ;
c) They are equipped with enhanced braking control, which makes them incompatible with other types of rolling stock, especially wagons ;
d) They are not capable of hauling other than their train insofar as weight is concerned except in an emergency;
So the power cars are not locomotives in the established and accepted sense.
The HST vehicles are not compatible with conventional LHCS, and cannot be intermixed, thus they cannot be classified as LHCS in the true sense, and their numbering reflects this.
HSTs are treated in a separate category because they cannot multiple with other MU trains.
Class 91s are locomotives because they were designed as such and are capable of working freight trains. They also have brake changeover switches as far as I am aware from memory.
DVTs are not locomotives because they are not capable of moving under their own power. Their power supply comes from an external source.
Does clear up the continuing debate ?
--- old post above --- --- new post below ---
Explain why Sprinters have alternative speed limits to other DMUs then
This is solely down to their enhanced braking capabilities.
