Actually, Propane DOES have a flow restriction. It isn't that it will decide to go all asplodey on you, though. The propane will actually freeze solid in the bottom of the tank if you ask for too much flow and then you have to wait for it to melt. In an application like this, I'm not sure you can actually hit that limit, though.

I think even the smallest acetylene set-up will cut through railroad track with the right tank and tip.

http://www.hoopersupply.com/tipchart.html

There's a tip chart. It's for acetylene.. but should apply to propane as well.. just need a little more patience with propane.