I will do my best to answer you without sounding too catty
>Having both locomotives on the head end seems like the easiest way to double head. Bridge weight limitations are also a reason to not double head this way or to stop and cut the lead locomotive off and have it run ahead over the bridge.
Perhaps on a Railroad where the bridges have not been taken care of. Remember, we are talking mostly about diesels here so the weight factor is a moot point.
>What reasons are there to insert a helper mid-train versus both on the head? How does this change operational practice?
It cuts down on the in-train forces like slack action and stretching over the road. In DPU (Distributed Power) setup, the second set can be controlled from the head end. It is possible for the rear to be pushing while the head end is braking. Not usually done, but it has been used while cresting hills. Also, in winter they are used as an air booster for the brake
>Why have a pusher on the rear instead of a double header? How does this change operational practice?
The rear set of units keep slack from running out and breaking the train in two as frequently as it could. There are tonnage limitations and length limitations which is why you see DPU units cut in 50 to 10 cars from the rear end on those ungodly long trains that they run now.
>If the helper is inserted mid-train, why this approach instead of running two separate trains?
SImple, One less crew to pay all the way across the road. In the case of the really long ones, two less crews to pay.
Listen carefully to a train that is using DPU. Many times only one or none of the mid train units will be revving up. This is due to the company's paranoia that they may be using too much fuel rather than delivering the train on time. Yes, there are restrictions on the use of the diesel power on the train.
Rick
Edited 2 time(s). Last edit at 05/18/2019 05:52PM by Rick Steele.
