"underground train" is a perfectly acceptable and usable expression in written English.
Example: I prefer taking the underground train instead of driving in the city traffic.
The Tube opened in 1863, the world's first underground train.
One meme unfavourably compared it to Moscow's underground train stations.
There's no underground train – which is a plus.
Underground train services are full to bursting point.
They would be able to fit on a single London Underground train - with seats to spare.
London's Underground train system, the "Tube," covers some 250 route miles (400 km).
An underground train line has also been built to link the centre to Sydney airport.
An underground train will begin service at Dulles International Airport on Tuesday, Jan . 26
But of the region's three major airports, only Dulles has an underground train.
Improvement of Tehran's underground train network would help alleviate the automobile-related pollution in the capital.
Rush hour found me standing near a young couple seated on an underground train.
