I wouldn't be too concern with the 1.5 release making some mods obsolete (i.e. Railcraft). Every single time Mojang released a "Mod-wrecking" update, it has always been an ***extremely*** watered down version. (i.e. enderchests ( only one type of chest with no way to set any select "Frequency" (i.e. Everyone on the server has the same shared inventory) and **no Ender Pouches!!!), just to name on off the top of my head. There are others) I 'Never' use vanilla EnderChests. Why use something that the original mod makes obsolete since before the update itself even came out?
So don't fret over the loss of RailCraft, I'm sure that RailCraft isn't going anywhere. RailCraft offers way too many things to become obsolete by a single MC update. Mojang may try to kill off some mods, but try as they might, they'll never match the quality of the original mods. And that is what we as the people who use these mods love, the quality of the mods, the intricacies and possibilities that these mods add to the whole vanilla experience. It is that that Mojang cannot match. They have created this fine game, but the modders have given it a dimension that cannot be equaled by any updates that coud potentially come out.
Just my thoughts. Sorry for being so long-winded.