A train goes up a pass, over a hill or mountain…
Often, there is a set of helper engines on the back or in the middle of the train.
If the helpers are on the back pushing, isn’t there some point somewhere in the middle of the train where the cars in front are being pulled and the cars behind are being pushed from the back?
That point is referred to as the “node”. Its location in the train at any given time depends on the forces being applied to the train.
And your point is?? To be realistic at anytime a train is moving you have front pulling and the rear shoving even if there isnt engines back there.
I started with wondering how long of a string of cars is safe to PUSH.
I know, it depends on ‘factors’: How tight the corners are, how many corners, how steep is the incline, how fast they are going…
Why do they sometimes have 4 engines on the front and 2 in the middle and 2 on the back. When they get over the pass, do they stop and disconnect the ones on the back and take out the ones in the middle, or just leave the ones in the middle. Just seems like if they have engines in the middle, it would be just about the same as just having 2 trains but they are connected together. Are the engine controls linked? Computer controlled? Is there somebody in the middle engines? Do they know approximately where the node is?
This is another case of trained and learned engineers running trains. You don’t just turn on the ignition, put it in drive and press on the accelerator except maybe the first day. After that you know the ups and downs and sideways of each piece of track and the push and pull of every size and type of train on any given inch of track at any given time in any given weather. There are engineering and operating standards you must know to apply to each situation or not apply in a given situation. If you are sitting on the right side of the cab you have your fingurs on the controls and your seat on the train.
Some years ago in Trains there was a recollection by or about then-President John S. Reed of the Santa Fe about his days doing locomotive tests with the dynamometer car. He recalled running over the tops of the boxcars - this was even longer ago than that out on the railroad, when there were still roofwalks and the crews were allowed to and required to use them - and looking down at the couplers to see where the slack was bunched and where it was stretched out, so as to determine where the ‘node’ was and which engine was doing how much work.
In the early days of manned and/ or remote radio-controlled mid-train helpers - like in the 1960’s - 1980’s, the typical ‘rule of thumb’ was to put about half the power about 2/3 of the way back in the train. That way, they would usually be pushing a good fraction - like 1/6 - of the train ahead of them. That seemed to work out pretty well in practice, from the standpoint of controlling and managing the slack.
Now, with the Distributed Power Units or DPUs, many different combinations and practices are possible. I don’t know that there is a standard or consistent rule any more - it seems to vary with each railroad, and with the division or territory or even the type of train. If you search for ‘‘DPU’’ here or elsewhere, you’ll probably find a lot of posts or ‘links’ that describe many operations in detail.
P.S. - See also the Trains article on the KCS from the 1970’s, which described the origins and modificatons of such a rule.