Fishplates with bolt holes at 5" centres are intended for flat-bottom rail. Do they fit ok on bullhead? They are 20" long (as opposed to 18" for bullhead) so wouldn't easily fit between the chairs at a bullhead rail joint. Flat-bottom rail fixings do not obstruct the fishplates to the same extent, and the fishplates can be longer. If an 18" fishplate is drilled at 5" centres, the end holes would be very close to the end of the fishplate.
We need the full info on this because Plug Track will need it's own 3D printed fishplates. The standard H-section locking fishplates from the trade won't work because the rail needs to slide into them. Whereas Plug Track rails are pressed home vertically. The fishplates will need to be attached to the side of the rail rather than inserted from the end. The insulator lug on the back of the fishplate will occupy only half the rail height (and can be cut off where the fishplate is on a dummy joint).
My experience of the 2 sizes of fishplates on the ESR is that they fit ok against the fishing surfaces of 95lbs bullhead rail, I don't recall any problems with a direct replacement of broken fishplates with the wrong pitch of holes with the 2 centre bolts, only the inability to fit one or other of the outers.
I don't think that a lot of time or resources were expended by BR on the branch in its final goods only days and the Extension from Merrifield Lane to Mendip Vale was done with recovered materials from various sites and was just cobbled together, sufficiently for the inspecting officer from whatever department it was at the time.
There's an ongoing replacement with bullhead rail and concrete sleepers from the loco shed access crossover towards Merrifield Lane, which includes replacing some very old 85lbs rail, the only 60ft lengths on the original part of the line.
I realise that this may have opened a can of worms, but can heritage be termed as being prototypical? We only used what materials we had available to us.
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