Funnily enough this question came up recently in one of the railway magazines.
It appears to be a detached HST▸ power car on the Newquay branch.
Can't remember exact details but unit failed and one of the power cars was detached and the passengers ferried to ?
There was a picture of it standing with passengers milling round I think at St Dennis or one of the other intermediate stations.
However, in general terms I still think single 153s on the mainline between Exeter and Plymouth are a complete waste of valuable train paths.
This is when an HST derailed near Luxulyan, as the location is on a high embankment, it was unsafe to detrain people, so they used the power car that hadn't derailed to ferry the passengers to (I think) Luxulyan station, as for 153's between Plymouth and Exeter, if they didn't run then Ivybridge wouldn't have a train service!
I have no wish to deprive Ivybridge of its train service. In fact it's just the sort of station whose use should be encouraged to help reduce local road congestion around a major regional center in this case Plymouth.
Regarding the 153 I was basically commenting on the ludicrous situation the railways are in regarding lack of stock that the only units available to serve Ivybridge are 153s and the presumably
SWT▸ don't have enough 159s to run hourly Exeter to Waterloo and do fill in runs from Exeter to Plymouth.