This is the 5510th most frequent Russian word.
поездом
"By train."
In this sentence, 'поездом' is used as an instrumental case noun to indicate the means of transportation.
Мы путешествовали поездом через страну.
We traveled by train across the country.
Here, 'поездом' functions similarly in the instrumental case to describe the mode of transportation.
Она ехала поездом в Москву.
She was traveling by train to Moscow.
'Поездом' again demonstrates instrumental use, specifying the method of delivery.