Wowak, I don't know the regs regarding pushers on passenger trains. I've never checked because it would be highly unusual to "push" a passenger train. I doubt that they would be allowed. The diesel that will be on the train under discussion here will not be providing any power. It will be just along for the ride going up the hill.
The reason you see pushers on freight trains is because the amount of tractive power required would exceed the rating of the couplers if all the power was placed on the head end. So some of the power has to be placed elsewhere in the train to distribute the pulling forces so you don't pull the train apart. In most cases this takes the form of a set of manned pushers shoving on the rear. Passenger trains are never heavy enough to present that problem, which is why you would see the "helpers" in the steam days on the front of the train, double-headed with the road power.
Out west, "Distributed Power" is often used to conquer the grades in the Rockies. This is a setup where a consist of mid-train, radio-controlled helpers provides the extra power. I have never run a train with DP, but Wyhog has. Perhaps he can chime in here with some additional info about DP.
