What is the difference between 乘 and 坐?
乘 = to board
坐= to sit (in)

坐火车= sit in a train (take seat on train )
乘火车= board a train
坐巴士= sit in a bus (take seat on bus)
乘巴士= board a bus
You can use '乘' , '坐' or '乘坐' when you take or ride any transportation that you can board and has seats.
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We use 騎 (mount/ ride) when the transportation has a saddle instead of seats. For example, horse and motorcycle.
騎馬 (mount/ ride horse)
騎摩托車 (mount/ ride motorcycle)