Guy showed me a glitch...
If you're low ranked like me it's very handy for grinding cash and xp..(level unlocks at rank 20, so may need someone else to start it)
On rail yard survival, where the T part of the map opposite the main gate where cops come in, there is a wooden crate next to the concrete barrier, you want to get on crate, go into stealth mode, drop onto barrier,(can take a few tries) walk across towards outbuilding with steel roof, jump up, when still in stealth got to middle of roof facing wall, when a decent distance away from wall push climb (x on Xbox) and you should climb through the wall. Inside you can shoot out but not be killed..
10 rounds, 30k for completing and decent Xp.. Normally rank up every go/every other