My recent experiences with DEF / SCR equipped Class 8 tractors taught me that the engine uses a great deal more DEF fluid in stop & go heavy traffic than out on the open road. This seems a natural to me, as the heavy throttle / idle cycles are more polluting to begin with. ( Obviously this would impact commuter train operators more than Amtrak. ) Also, the DEF fluid tanks on trucks have an integral pump / heater unit in them. Our local fuel jobber began the DEF fluid re - filling deal with a 250 gallon plastic tote in the back of a pickup truck with an electric pump on it !!! I understand most fuel jobbers now have a separate DEF fluid tank on the diesel re - fueling rig, though I don't know its' liquid capacity. As DEF / SCR equipped Class 8 truck tractors are becoming more commom, I'm guessing Amtrak won't have many problems finding DEF - equipped jobbers most anywhere in the U.S.
The one major point that was emphasized to me constantly is to NEVER let the truck run out of DEF fluid!!! I've been told it's worse than running out of diesel, in terms of being a pain in the @$$ to re-set and even re-PLACE the emissions controls afterwards.