I'm assuming they're one's brought by FGW, & are awaiting dispatch up north for refurbishment. For some unknown reason, when FGW buy these coaches, they bring them down here & dump them in a safe place in the yard, until a Cotswold Rail loco comes to take them back up to Bombardier Derby. Bit silly if you ask me, bringing them all the way down here, just to go back up again when ready. The other explanation is they might be for the Virgin weekend workings to & from Newquay, & are a full HST set with some spare cars just in case. I know they do keep a MML set here during the week, & it gets swapped over for another during it's travel north, if that makes sense. It's also used as a standby set, just in case any probs occur with the Voyagers when they come down here. The train is usually kept in the main sidings opp. Embankment Road, the main roud out of Plymouth that runs alongside the depot. Where I am, I can see Lipson Junc., so to see above the depot itself, I just talk a short walk across, & have a clear view of the line from there. It gets a bit busy at night, as it's used to bring various stock, Voyagers, Adelante's, & HSTs, into Laira depot for the night.:grin:
Mart
[EDIT]
Btw, the same situation goes for the power cars as well.
Mart