153s and 155s (same thing as far as gauging purposes go) are prohibited because the body-side steps under the doors foul the platform coping stones. These steps cause problems and every now and again we hear of new incidents at other locations where the steps on these units have struck the platform edges, there are also places where these units ar normally cleared but if their secondary air suspension is isolated they're prohibited. IIRC MkIII stock is restricted/prohibited through Wallgate too, also class 175, 180, 185, 220 & 221.
I should point out that when the 155s were withdrawn in 1989 pending door mods, they were all sent to Steamtown Southport for the modification work to be carried out so no doubt they would've gone through Wallgate to reach Southport...unless that rule doesn't apply to ECS movements?
Just as well 153s are banned through Wallgate as the Southport line services do get rather cosy at times even with a single 2-car unit.
If it counts for anything, 158s are cleared for Wallgate as well.