Several parts combine into one character.

汽 is a phono-semantic character: 水 (氵, water) carries the meaning, 气 (vapor) lends the sound. Water (氵) rising as vapor (气) — steam. 气 was originally the shape of rising air or mist, and here it serves as both sound and sense.

Korean hanja-sound "gi": 汽車 (gicha, train), 汽船 (giseon, steamship), 汽笛 (gijeok, the whistle of a train or ship), 蒸汽 (jeunggi, steam). The word 汽車 (train) came from the steam locomotive that ran puffing out vapor.

Mandarin: qì, 4th tone. 蒸汽 (zhēngqì, steam), 汽车 (qìchē, automobile — "the vapor-driven vehicle"), 汽油 (qìyóu, gasoline), 汽水 (qìshuǐ, soda / pop). Intriguingly the same word 汽車 means "automobile" in Mandarin (qìchē) but "train (steam locomotive)" in Japanese and Korean.

Japanese: on-reading キ (ki) — there is no kun-reading (this is an on-only character). 汽車 (kisha, steam locomotive), 汽船 (kisen, steamship), 汽笛 (kiteki, steam whistle). Japanese 汽車 (kisha) refers to the old steam train; today's electric trains are distinguished as 電車 (densha).

Memory aid: water (氵) rising as vapor (气) — the steam that surges from boiling water.