I saw this announcement on train:
The train service terminates at ...
I think, in the noun phrase the train service, the word train does not modify the noun service (Edit: as other noun acting as Adj ).
I think, tt should be the service of train; therefore, the sentence called for a genetive ,with 's, not noun acting as a adjective
the word train should have the 's to show a possessive.
(Edit) The train in the question is not a regular service, it's an ad hoc service in rush hour.
What should it be?