Someone is in love with a friend who doesn't feel the same way, and "the language of love" is the speaker's image for that gap. The speaker speaks it fluently, but the friend simply doesn't understand it. Each time the speaker reaches out, whether by holding a hand or trying to show their feelings, the friend answers with a polite, puzzled "my friend," a phrase that keeps pushing them back into the role of friend.
The speaker feels lonely and a little resigned. The line "I wished that neither I spoke the language of love / But I do. And you don't" sounds tired rather than angry. The train scene makes the distance physical: the friend waves and smiles from the other side, the speaker can't smile back, and the station takes them out of sight. At the end the speaker gives up, waves, and tells the friend to forget it. The mood is quiet, wistful, and gently painful, the ache of a love left unspoken.