Thanks for the replies so far.
Looks like it’s most likely that the lines are named after the district name: I guess naming after that Atlantic type of loco is plausible, although this would only be if the line was built around 1900-1920 when such locos were in vogue. The SE&C, the lines' builder, was known for large tanks, not sure how many Atlantics they had.
Definitely not the Atlantic coast express, as that run via Clapham, Working, Salisbury, Exeter and towards north Cornwall with various through portions, seen here in 1960:
[youtube]3Ob3qUhK0BE[/youtube]