You've all got me wondering now about that 'mineral line'. I've always assumed it meant a mineral railway, one of Sir John's favourite
themes.
Now I'm not so sure, as the 'soft drink' theory sounds as plausible, and makes more sense. Still, as theleveller says, perhaps the line is meant as symbolic rather than literal.
In the Betjeman Collected Poems, JB himself includes a kind of 'stage direction'for this poem:
'N.B. This should be recited with a Midland accent'.
When Sir John recited it himself on 'Banana Blush' (with a musical setting by the always excellent Jim Parker) he did it in a kind of generic Northern Stage-Yorkshire which is completely wrong, but very engaging.
Not a criticism, you understand; Sir John could do no wrong in my eyes.