Most bridges have been rebuilt on a “like for like” basis, as the Local Authorities have enough call on their funds without providing extra for bridges that wouldn’t have needed replacing but for the electrification works. If you went across the old Kerse Road bridge, it was pretty much life expired. The council have got a cracking new, higher capacity bridge for a lot less than if they had replaced it themselves, which they would had to do in the not too distant future.
Low-height deckers for this route have been a thing going right back to Strathclyde Buses' days. Low-height Olympians were standard for a number of years, then the tri-axle East Lanc Nordic-bodied Volvo B7Ls arrived. These, in turn, were replaced by the current lot of low-height E400s in 2011. As the last linked image clearly illustrates, it's a very tight fit, even with the vehicle built to a lower height.
There must have been a lot of triple and quadruple checking of the heights of those E400s before the first one tried to run under that bridge!
Rail bridge obviously built before double deckers and lorries were thought of.
Have a look at slide 30 in the presentation referenced here Electrification threads and resources index in post 76 (sorry, not good at this linking stuff).
Quite a lot of people get off at intermediate stops which makes matters worse as they are often stuck in the middle of a coach. There was a fatality on the bridge a couple of years ago when an HGV, trying to negotiate the bend, mounted the pavement and crushed a pedestrian. Plenty of other less serious incidents too and hazardous for people trying to cross.
Here’s a better view of the bridge.
Excellent news; I hope that we can start seeing electric locos on freight out of there soon.
They've less need to, now that more 385s have been delivered and that the 380s have gone back to Ayrshire workings.