Some transport uses ‘in’. Some transport uses ‘on’.

The rule for this is not whether you can go inside the vehicle.

The rule is:
services use ‘on’
independent or special transport uses ‘in’

(An easy way to think about this is: does the transport usually do this journey, even when I am not here? If it does, it probably uses ‘on’.)

In

On

in the taxi

on the train

in the boat (if a private boat)

on the boat (if a ferry or service)

in the plane (if a private plane)

on the plane (a normal plane)

Exception

Bicycles use ‘on’.