For anyone who either likes to build spread out or "Railworld" style like myself, one of the first problems you run into is how to refuel trains that don't all arrive at some central point.
1st Setup a place to make fuel. For me this is my PetroChem facility. I start usually by mixing extra petro into blocks. Later I build Liquefaction to ramp up production for Rocket fuel. A 1-1 train should be sufficient for most networks to handle deliveries.
2nd Identify places of common intersect. For me this is my Home base, Science Base, Ore Smelting Yard, and Nuclear depot. Here I create a Fuel specific station for drop off and 1 roboport (or enough to cover all stations at this base)and a single stack of bots (more depending on the amount of trains) It really doesn't take many. Rocket Fuel burns sloooooooooooooooooooooow.
-Limit the size of your drop off chest to a reasonable number. I chose 100 Rocket Fuel
-Wire the output chest to your station. Station should enable when fuel is less than some number (100 Rocket Fuel in my case)
-Requester chests are set to a full train load of fuel (30 Rocket Fuel or 150 Solid Fuel depending) though you could easily go with 1 stack of fuel (10/50).
Wait conditions on the delivery train are flexible for fuel pickup but probably best at "Empty or Inactivity=5" for drop off.