Not sure if it's the same south of the border but up here they are meant to. In my experience if it's a local/suburban train the replacement will be a commuter type double decker bus and you'll have a hard time, further afeild they use coaches and likelyhood of getting it on is a lot better.
I taken my bike on a rail replacement bus from Horsham to Chichester before, it was one of the local double decker buses. Just propped it up next to the driver. I think he took pity on me as it was about 22:00 and pissing it down.
Not sure if it's the same south of the border but up here they are meant to. In my experience if it's a local/suburban train the replacement will be a commuter type double decker bus and you'll have a hard time, further afeild they use coaches and likelyhood of getting it on is a lot better.