Fill in the blank with appropriate preposition:

We travelled___ the 6.45 train, which arrived at 8.30.

The correct preposition to fill in the blank is by.

The sentence now reads: "We travelled by the 6.45 train, which arrived at 8.30."

Here's why:

• The preposition "by" is used to indicate a method of transportation.

• In this context, "by the 6.45 train" means that the people used the 6.45 train as the means of transportation.

The other options, "through" and "in", are not appropriate in this context.