Questions & Answers about Le dixième train arrive.
Le means the, so it points to a specific train: the tenth train.
With ordinal numbers in French, you very often use the definite article:
- le premier train = the first train
- le dixième train = the tenth train
Using un would sound unusual here, because tenth normally identifies one specific item in an ordered series.
In French, ordinal numbers usually go before the noun:
This is different from many other adjectives in French, which often come after the noun. Ordinals are one of the common types that come before it.