Ray: there is a way around that problem You need to use a DC to DC Boost converter to boost the 4.8V to 5V. you can find these on Evilbay all the time and are quite small and cheap. Here is one that I found that would work.
DC-DC Step Up Boost Converter 3V to 5V 1A USB Charger Mini Mobile Power Supply
Ebay Item number:370790917931
$3.50 and is 24 mm x 18 mm so it should be able to be place just about anywhere. This is design to be used for MP3 players, so the output connector is a USB connector, but on this version it is surface mounted, so it it is easy to un-solder and remove. Then all you have to do is solder your power wires to the two output power pads. These would be the two outer ones of the 4 pads where the USB connector was removed. When looking at the top of the USB connector when you can see the 4 pins, the Positive is on the right and the negative is on the left.
Should take a whole 5 minutes to remove the USB connector , add wires and install in your railbus.
I use the Step Down versions of these all the time and they work great. At this price it is worth a try, don’t you think. The only drawback to getting these is that most of them come from China, so it takes a couple of weeks to get them after you order them. I usually buy them 10 at a time so I have them on hand when I need them. I haven’t ordered any of these yet, but I probabbly will.
Dan S.