I used the product from inland marine that Jack suggested, it worked out pretty well, I did have to treat one of my thwarts more then once, but it fixed the problem.
A bottle of the stuff for under $100, compared to $1500 for the new floor, doesn't sound like a terrible gamble to me, though the I beams may prevent it from working, as already stated.
There are products you can paint on the outside, though I have never used myself.
If you got desperate, and it was a new floor otherwise, wonder if you could take a hypodermic needle, fill each section of the floor, then aqua seal each of those tiny holes, in theory the sealing compound would help fix those tiny holes as well. Obviously a risk to that idea, but if your about to drop $1500 anyway?.........
" I wish I were a headlight, on a North bound train..."