Normally, when a powered rail runs into a block at one end of it, a minecart that is placed on that powered rail will accelerate away from the block. Similarly, a minecart that runs into the block will then accelerate away in the opposite direction. This doesn't happen when the block is the back side of stairs, even though the backs of stairs behave like solid blocks in other ways, like blocking light, connecting to fences and having torches placed on them. See attached video.
This also doesn't work with leaves, glass, and other transparent blocks, but I'm not sure if that's intended behavior.