English Translation
I have to catch that train.
乗らなきゃならない sounds like a bit old-fashioned male talking.
Other expressions are:
- あの電車に乗らなければならない。(in a more formal and decisive tone)
- あの電車に乗らないといけない。(casual)
- あの電車に乗らなきゃいけない。(more casual)
きゃ (Kya) is phonetically transformed from ければ (Kereba) so as “wanna” is transformed from “want to” in English.
ならない is “definitely have to”, which is much stronger than “have to”. You can find ならない more often in writings than in conversations. So, きゃ + ならない isn’t a common collocation.