The MSWJR crossed over, not under, the Swindon - Chippenham line at Rushey Platt. The connecting line made a double track junction. Looking at the 1910-ish map at
https://maps.nls.uk/geo/explore/#zoom=16.0&lat=51.55382&lon=-1.80979&layers=168&b=1, then that location looks posssible. The signal post is shown in the right place on the map. There's a road underbridge beyond the junction.
I think that one of the connecting line's tracks is visible between the train and the crowd, but the other track is hidden underneath the crowd.