Probably empties heading back East, but diverted via Standedge due to engineering on Calder Valley perhaps? I seem to recall that happening on a couple of occasions, but like you, only saw them take direct route 'round the corner' (as we called it), rather than dive-under route. I'm guessing if they'd have been held at the signal for some reason, even a Grid would have had great difficulty restarting a full train of empties on the rising gradient, while most of the train would have been on a curved lines in and beyond the underpass, making starting even trickier? Perhaps an ex-driver could enlighten us?