OG Trainyard in World's Edge has always had a way for every character to get up to the hollow carts above; the circumstance you're talking about "not all characters can get there by themselves. There are not always jump towers. And you should not have to rely on having a particular character to get there." is already a thing on a base by base case, it's something a player should consider when picking characters expectedly for synergy with other teammates. Don't know if you realized this, but this is a team-orientated game and only recently been giving solo players wiggle room.
Using your example, everyone now can get onto the carts in the train yard fairly easily, but what about the framework above the carts above. Pathfinder can place a zipline to get up there, Valk can rocket jump her crew on top of there, so why can't you stay up there? Or stay on top of the mountains to avoid taking tunnels as they are notorious kill spots?
And with that said, why give certain characters the ability to get to places that are 'off limits? Why can pathfinder set his zipline in places considered out of bounds?
Overall, removing the upper bound gives us players more options; removes bugs from the game, adds areas for them to hide Easter eggs, its a win-win-win; there is no good reason to keep it. In your case, where you have no teammates that can get to higher places ( which you can with jump towers, so memorize the map, and plan ahead ) OR your team synergy and strategies should reflect what you're good at. So a Crypto, Lifeline, Gibraltar team should be focused on CQC rather than high ground, open areas. All of them should use mid to close-range weapons as that's what their abilities work with, rather than pathfinder or Valk, who can make space easily and can easily use snipers and other long rang weapons.
