Given the fact that many non-local travellers will come from, or be used to, areas that have both trams and metros, to use an internationally recognised pictogram for a tram to point the way to a railway system that is not, and is never referred to as a tram, would be just as confusing as using the local symbol/icon. "But I'm looking for the Metro, not a tram..." The tram symbol would be fine for Manchester, Birmingham , Sheffield, Edinburgh, Nottingham and Blackpool - but Newcastle and Liverpool need something else. How fortuitous that both systems start with an "M"! That just leaves Glasgow - but with only 1 "in-station" interchange anyway and a fairly logical "S" for "Subway" it's not that much of an issue