It literally is just momentum and getting the "trick" of each gap to work. For instance: From the Car/Truck rail to the rail that has the phone booth in between, you have to come fast ("boost" button at the beginning of the rail) and do a wall jump on the billboard.
Now, the trick to do the two cabs/billboard grind all the way to the previously mentioned rail, you gotta come fast from the catwalk, jump directly into the billboard, wall-ride it, jump off of it, land on the rail after the cabs. If you did it right, you'll land on this rail near the end of it - enough that you can connect, grind a second and jump off near the endge. Then you'll be fast enough to cover the distance to the other rail.
With those two key moments, you can loop the entire stage and, if you don't make a mistake and slow down, you can do endless tricks. I got the 100 tricks achievement like that.