In a conventional train, the "traction force" is established at the head of the train,?

Question

the engine, why not put another traction force at the end of the train, burning fuel like a "jet" and exiting at the tail end like an "inverted Y" which will "lighten the load" on the head end, so lesser traction force is required to pull the weight of the train, it is something like pushing a vehicle from behind to make the engine move at the head end...? I checked Bombardier JetTrain but I am not convinced that the Total force (traction power) required to make the train move anyway, will not change, it will only split the power in two halves (one at head, one at tail). Is there scientific proof, that putting a traction power (exhaust) at the tail end of the train, improves fuel efficiency or speed of the engine of the train?

Answer 1

Though not jet powered, they do use engines either cut in to the train or pushing from behind.  These are collectively referred to as "helpers".  

Helper engines are not a new concept.  As you pointed out, the engine on the point of the train, the "road engine", can only pull so much tonnage.  Adding additional power to the road engine reaches a point of diminishing returns due to the limitations of the "draft gear", the components of the coupling system, including coupler, knuckle and draw bar.  The safe maximum force the draft gear can handle is 240,000 psi, reference to the knuckle face.

When this situation arrises, the only option is to "split the power", as you pointed out in your question.  A helper engine will be placed somewhere within or behind the point where the trailing tonnage will exceed the capability of the draft gear.  But there are things that must be considered when determining where to place the helper.

Helper engines have, can and will push cars ahead of it off the rail, which is called "jack knifing".  It took experimentation to find out that when a multiple unit helper is entrained, it must be placed such that it will push 1/3 of the tonnage it handles, and pull the remaining 2/3s.  Keep in mind, the road engine is still pulling at its limit, while the helper handles the ballance of the tonnage.  Ahead of the helper there will be a point of neutral slack, where the last car being pulled by the road engine is coupled to the last car ahead of the helper that is being pushed.  This is called the "node".  The node shifts, as the the overlap in tonnage handled shifts during speed changes or changes in terrain or resultant slack action.

Helpers consisting only of a single unit or two smaller units can be placed behind the rear of the train.  On some monster freight trains, a helper "cut in" to the train is required along with a helper behind the train.  In this situation, the helper cut in is referred to as the "swing" helper and the other helper is denoted as the "rear" helper.  This situation arrises when the trailing tonnage behind the swing helper maxes out, just as behind the road engine, requiring the rear helper to relieve this force, creating a second node behind the swing helper.

Obviously the engineers of each engine must all be on the same page and coordinate their train handling to maintain control of speed and slack.  These days some railroads are using radio controlled helpers that are run from the road engine by the engineer on the point.  This is usually referred to as "distributed power".  

Though economical, my personal feeling is that these are dangerous in practice.  Being radio controlled, often times radio signals are interrupted by any of a large number of factors.  When this occurs, the DPUs (helpers), rather than shutting down creating a train handling problem of a major proportion, they will go into "over ride", where they will continue to operate at the last command they received for a period of time before automatically throttling off, gradually to idle.  

Which means, if they were in full power before losing radio signal they will continue to operate at full power until the over ride times-out or radio contact is re-established.  If conditions change during this interval, the only way the road engineer can shut them down is to "pull the plug", which means placing the train into emergency.  This is EXTREMELY dangerous.  

The Unions couldn't keep this Frankenstein technology from being used, but I would urge all to contact US Representatives, FRA, DOT, DHS and NTSB and bring this non-sense to an end.  Especially in a climate where these rolling bombs can be turned against us.  "Radio signal."  Get it?

Using helpers doesn't improve fuel efficiency but does increase attainable speeds and allows heavier trains to be operated, which improves economy, to answer the last of your question, which is a very good one.  Thanks for asking.

Answer 2

One thing you need to bear in mind is that a train is not a rigid body, not a single vehicle, it's a series of rigid vehicles with couplings that allow for some movement.  This movement is a vital factor determining the power required to get the train moving, which is when the maximum locomotive power is required. The power of multiple locomotives at different points along the train therefore do not simply add up, although multiple locomotives at the same point, whether the front, middle or end, would add up.

Answer 3

ask a train engineer . most trains have at least two engines, usually at the front , but occasionally a rear engine is attached but may only be used in a switchyard, but i'm not sure.

Answer 4

On GNER Mallard trains the streamlined engine is always on the North of the train (there are exeptions usually through faullts)

So going North the train is pulled by the locomotive and going south it is pushed with the driver having a cab in the Luggage Van

However this is not aways the case as if there is a fault in the remote control system then the locomotive will be moved to the south end and will haul the train 'blunt end' first at reduced speed (110 mph instead of a potential 140mph)

Also it is common for fast passenger trains to have a locomotive at each end, and on newer trains there is distributed power which means placing motors on the coaches, examples being the German equivient of the TGV and the high speed DMUs in the UK (with a 750hp engine under each coach)

Answer 5

Actually they do put power at the rear of trains this process is called distributed power most heavy hall trains 19000 ton or more are equipped this way, these units are run remotely from the head consist to mimic all action performed on the head end of the train. The trailing tonnage actually helps the wheels maintain traction,once the initial movement is made the rest is just well inertia. As far as your fuel efficiency question I really don't believe that the efficiency to the whole train improves,yes there is less fuel consumption with each individual engine but as a whole the group would still consume the same amount of fuel!

Answer 6

I am not sure about that but a few times I have seen long freight trains with extra slave locomotives the middle of the train. I don't think a rear end pusher locomotive would be good on a long train. I think it would tend to push the cars of the rails by buckling the line of cars. I am likely totally wrong about this.