(17 Jan 2017, 8:16 pm)mb134 wrote That would leave 2 vehicles that aren't famed for their reliability being relied upon as spares? 

The whole point of getting 7501-9 away from the X21/22 would be to improve reliability. To maximise the probability of this, you'd give it a new allocation of vehicles, in addition to 7522-8.

But a new allocation would totally cock the image of the service with a mix of classic Enviro 400s and MMCs. Mixed fleet allocations don't look good on premium services and that applies to any operator.

What if another suitable vehicle type could be brought in as a spare?