I used the London to Bristol train for the first time for a long time yesterday, it's a great route for seeing plenty of deer on virtually the whole journey and it was a nice time to look for deer leaving Paddington at about 16:45 just as the light started to go.
Just between Didcot Parkway and Swindon I saw about 35 roe in about 5 minutes on the north side of the track. They were mostly in family groups of three or four but in one field there were at least 12 animals but the train passed before I could get an accurate count.
Now I wonder who has that ground.......?
I have also seen deer ( fallow and muntjac) on a number of occasions on the Euston line, fallow not far from Beaconsfield and muntjac around Leighton Buzzard and another on the embankment outside Watford station.
Are there any other railway lines where you have a good chance of seeing deer?