how to say “subway” or “underground train” in Hebrew
רַכֶּבֶת תַּחְתִּית
The word תַּחַת 
listen and repeat means below or under, while מִתַּחַת listen and repeat means underneath (literally, from under) as in:
listen and repeat means below or under, while מִתַּחַת listen and repeat means underneath (literally, from under) as in:הַנְּיָרוֹת נִמְצָאִים מִתַּחַת לַשֻּׁלְחָן.
The papers are under the table.
תחת also means one’s behind in slang.
תַּחְתִּית listen and repeat means the bottom or lower part of something, so that the part of Haifa at the foot of the Carmel is called הָעִיר הַתַּחְתִּית listen and repeat – the lower city. And a subterranean train is called a רַכֶּבֶת תַּחְתִּית listen and repeat, or simply תחתית, as can be surmised in Ehud Banai’s song (here’s the music video, and here are the lyrics) about Brooklyn, NY that combines both these uses of תחתית:
listen and repeat means the bottom or lower part of something, so that the part of Haifa at the foot of the Carmel is called הָעִיר הַתַּחְתִּית listen and repeat – the lower city. And a subterranean train is called a רַכֶּבֶת תַּחְתִּית listen and repeat, or simply תחתית, as can be surmised in Ehud Banai’s song (here’s the music video, and here are the lyrics) about Brooklyn, NY that combines both these uses of תחתית:בְּרוּקְלִין שֶׁל מַעְלָה, בְּרוּקְלִין תַּחְתִּית
הָרַכֶּבֶת עוֹצֶרֶת רַק בַּתַּחֲנָה הַסּוֹפִית
Brooklyn of on high, Brooklyn of below
The train stops only at the final station