Like many heritage railways they have come to the realisation that their main leisure business doesn't require ever longer runs alone with associated costs and operating difficulties manning additional signalboxes etc. A specific objective like the GWSR's Broadway project can make sense however where it adds a particular popular destination or otherwise plugs into an additional market.The ability and appetite for the WSR to run direct have both now diminished.
I'm sure the cable route, a number of signal structures and equipment cabinets were erected foul of the old up relief alignment. I was an S&T trainee at the time and saw their construction. A new independent alignment might have been possible further out within the railway fence, but would have required a lot of work creating it's foundations. I don't believe space under the Silk Mill bridge is an issue. The steel truss bridge has no intermediate pillars and spans a massive 42 metres or thereabouts between its abutments (measured on Google Earth). The mains may have moved a little but not significantly enough to make any material difference.The dedicated route would have still fitted after all the re-signalling, until they built Silk Mills Bridge and the support post was put on the available alignment on the Up side. The lines have also been slewed for higher speed running in the last 42 years