I'm pretty sure there isn't many, if any 158s at Waterloo off peak, but I could be wrong
Was it a Friday by any chance?The 15:50 arrival at Waterloo (Ex-Bristol TM/Exeter St Davids) was a 158+159+159. The 158 was joined at Salisbury when I was on it not long after last week.
the xx49 arrival forms the next xx20 departure
the xx19 arrival forms the next xx50 departure

The 15:50 arrival at Waterloo (Ex-Bristol TM/Exeter St Davids) was a 158+159+159. The 158 was joined at Salisbury when I was on it not long after last week.
As 'Southern' say's the 1620, the above *might* form the 1620, however when I got to Waterloo there was a 3 car 159 already in the platform next to us which might have done it.