Steel anvils only let you go up to level 39, like vanilla anvils while they are supposed to let you go up to level 50. They are also supposed to be more durable but I have not tested that, they might be or they may function completely as vanilla anvils.
Whether or not this is true. This is a bug to be report to the developer of Railcraft (CovertJaguar), and not something that the MPT could fix AFAIK.
I looked up on the railcraft update history and it said "Added steel anvil which has increased durability and allows up to level 50 enchantments" so you know you would think he woulda tested it but yeah if the modpack guys don't see anything I'll bring it up there. Thanks. It does the same thing in single player, creative and survival: