。
The wait for the train is long.
Literal
Train [of-の] wait-time [subject-が] long.
待ち時間 ('wait time, waiting period') is a compound: 待ち (the い-stem of 待つ, 'to wait') + 時間 ('time'). This pattern of verb-stem + noun is highly productive in Japanese for forming noun compounds from verbs — 読み物 (reading material), 書き方 (writing style), 使い方 (how to use). Here が marks the subject of the い-adjective 長い.