Northern Railways

As already stated, there isn't any 90mph running on the North Clyde at present. Seeing as they are going to be working Lanark services alongside the 318s soon I presume the yaw damper fitment is to allow for 90mph running along the WCML in order to presumably avoid retiming Dalmuir-Lanark services to increase the overall journey time to accommodate the 320's lower (at present) top speed. With the toilets as well, you could argue that they are effectively increasing the 318 fleet as they will be pretty much similar on spec to allow for a common fleet, therefore increasing fleet flexibility.

With Helensburgh Central in particular, the single line formation between Craigendoran Junction and the terminus would presumably hamper any attempt to provide a 15 minute frequency? Westerton-Milngavie isn't exactly an ideal route either with what was once a double track route now a single track route with a short double-track section remaining somewhere in the middle.