"up train" is a noun phrase meaning a train heading towards the main city (as opposed to away from it, which would be the "down train"); "up" is an adjective here. I guess this is what is confusing you.
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"up train" is a noun phrase meaning a train heading towards the main city (as opposed to away from it, which would be the "down train"); "up" is an adjective here. I guess this is what is confusing you.