X39 was only deckers from late 96 until early 99. Through the late 80s/early 90s, two bph were X39 with the others being X3 (mix of deckers and Nationals) and X4 (W&D Tigers and badgerline Nationals). From 92 to 96, the X4 was replaced by another X39. My notes of trips from the early 90s had Mk1 Nationals working the Sunday 339s! Mind, the 332 was a decker route but mainly because Eastern National/Thamesway donated a lot of spare VRs to help replace knackered Nationals
Half the Mon to Fri X39s are deckers, drawn from the eight Geminis and built around key heavily loaded boards. Where the Streetlites tend to suffer is when you have bunching in service where one is packed out followed by a half full one 5 mins behind. Certainly, a 15 min service with deckers would help that.
However, unless the Sunday 70 can be interworked with something else, you can't easily increase the frequency to 20 mins. Increasing costs by two thirds would be prohibitive as a standalone. If it's only "fairly full" on a day with a big event on, I can't see how it would pay to add two extra vehicles into the cycle; the 40 min journey time makes it really awkward.
That said, it is quite incredible how Sunday services have generally improved over time. The 376 had only 4 on a Sunday (with minibuses) whilst the 173 only had 3 trips. The 267 had nothing at all! However, the 231 still only goes two hourly whereas the 265 is now hourly.
As for e200s, they're ok for some routes but one of the worst journeys I ever had was Rhosilli to Swansea. Bangs and rattles on a fast, rural route that will last with me! Ok for 14s but they have appeared on the 267 recently!
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Just checked you're right - 265 is nine vehicles. So with Matt Splatt's revelations about the 184, it could be that Westbury has quite an allocation:
265 (9)
234/267 (6)
184 (2)
260 (1)
236 (1)
30 (1)
If that's all correct, then Westbury has a PVR of 20. That's more than TE depot had when it closed in 1983, and similar to WS! No wonder the Melksham to Bath service got moved to BH!
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For a 15 min frequency, you'd only need 8 plus a spare. Currently 10 plus a spare, hence 11 though with interworking, that's only a theoretical allocation at the moment