Preposition for transportation.

BY (general ways to travel)

The preposition ‘by’ is used when talking about general ways to travel.

For example: We can travel by train, by bus, by car, by boat, by plane.

It is also used to refer to the transportation environment (by sea, by land, by air) or on the surface area(by road, by rail, by water).

Specific Transport Vehicles-

We can use either ‘on’ or ‘in’ to talk about specific transport.

I am in car.
I am on car.
How do we know which to use?

Using the preposition ‘in’

If we can sit inside the vehicle, we can use ‘in’.

For example:
In a car.
In a truck.
In a taxi.
In a rowboat.
In a helicopter.

Using the preposition ‘on’.

If we can stand or walk on the vehicle, we can use ‘on’.

For example:
On a train.
On a tram.
On a plane.

We can stand or walk on all of those, even when they are in motion.

If we actually sit or stand on the top of vehicle, then we can use the preposition ‘on’.

For example:
On bicycle.
On a surfboard.
On motorcycle.
On a snowboard.
On a skateboard.

Walking (on and by)
We go on foot ( meaning as a general way of moving from one place to another). Or we go on foot.Therefore, both are correct ways to describe walking.