You can counter the railgun spam on Vortex Portal by taking control of middle, the only place (IIRC) where the railgun weapon and ammo spawns. This area is also important to control for the megahealth below and for being generally the fastest way to cap.
It's unsurprising that keeping a hold on this area is not an easy task, but if you can do it your team will gain quite the edge over the enemy team. "Railwhores" will need to confront you on your terms, but don't assume that just because they spend ungodly amount of times camping portals with the railgun it doesn't mean they're not proficient with the rocket launcher. (IIRC, it's trivial to enter middle with 25 rockets, while keeping your weapons stocked up while camping middle is trickier indeed.)
As for Rebound... I don't recall the railgun being as "overpowered" on this map. If you know the enemy is camping your flag pole (something fairly safe to assume), you might want to forego the quicker but more dangerous lower routes by taking the main route, or better yet the not-as-frequented lateral ones.
If you have to take the lower route, consider trickier ways to climb on the top like climbing your way on the vertical pillars with rocket jumps or plasma climbing. It won't work eight times, but it might just be the move that changes the game. Plus, you get obvious bragging rights for pulling this off in a real game, with people like shooting at you and stuff.
If you get knocked off the map, try your luck and drop the flag (you have bound that command, haven't you?). If you're lucky enough it'll fall downstairs for a teammate to grab. It's going to reset if you don't anyway.