I too also understood the 'mineral line' as a light railway. There's absolutely nothing in the first verse to suggest that the mineral line is inside a room - wherever did that come from? It doesn't even say whether the girl singing a hymn is in the Institute or outside in the street (or whether the gym is in the Institute building or elsewhere). And no-one is suggesting that the canal is indoors, so why should the mineral line be?

However, I don't visualise the mineral line as anything so sophisticated as in Paul's link - something more like this. The photo clearly shows the line being run by one man (and two horses). Apparently the line was horse-drawn until 1899.

But as an earlier poster said, it's poetry not a history lesson.

Andrew