Use zero article with 'by + transport' (by car) and 'the' with some specific transport (on the bus).
by + transport (no article) | on/in + the (specific) | in + a (one vehicle)
Don’t add 'the' after 'by' in the standard transport pattern.
Did you go by car or by train?
💡 Common: by car/bus/train/plane. Also common: on the bus/train, in a taxi. Many phrases are idiomatic, so learn them as chunks.
✓We went by train.
✗We went by the train. (general)
By + transport usually takes no article when it means the method.
✓We were on the bus when it started raining.
On the bus often refers to a particular ride/service in context.
✓I came in a taxi.
Taxi is singular countable → a taxi.