I've used a few beam saws in the past, but don't remember all the details.
Yes, you can butt rails end to end, but most beam saws use V-Rollers and track, or something similar to move the motor carriage. It's more immune to dust than linear rails, and it's cheaper. Consider chain drive to move the carriage. Cheap and simple. Or rack and pinion. Moving the saw blade does not require high precision. Are you going to have a scoring blade?