Not too sure about Mickle Trafford. I know there's the Millenium Greenway footpath along the old railway line that used to go to Chester Northgate from Mickle Trafford, but not sure how close you can get to the signal box. I think the path ends at a derelict bridge going over the existing Manchester-Chester line so you might not be able to get any further than that. Maybe better shots from Station Road in Mickle Trafford.
Best view is from a passing train with opening windows, which nowadays usually means a charter train.
I love that MOD story at Sealand!