I like this a lot - I have all my trains topped up with solid fuel, all I have to do is throw down the occasional new fuel stop as the network expands and make sure the top-up train visits it from time to time.
But...
If I want to switch from solid fuel to rocket or nuclear I need to do it everywhere due to stack size. I have trains set to get more SF when they drop below 12, then they top up to 150. (Three stacks of 50). Nuke fuel stacks in 1s.
Rather than refuel when less than <number> how about an option to refuel when less than <percentage>? Then setting it to 1/3rd or 1/4 would work with any fuel, even if different stops provide different content. Then switching over wouln't involve a mad panic to upgrade all the supply trains and empty out the old stocks form the stops.
