Learn German: Der Zug ist langsam. — Das stimmt. - The train is slow.

In German, grammatical gender does not always match physical logic. Even though a train is an inanimate object (which might make you think it should be neuter, das), the noun Zug is grammatically masculine. Therefore, it takes the masculine word for "the", which is der. You just have to memorize the gender along with the noun!

Why doesn't langsam have an ending here?

In German, adjectives do not get any extra letters added to the end when they come after the verb (like ist) to describe a noun. You only add endings to adjectives when they are placed directly in front of the noun they describe, like in der langsame Zug (the slow train).

Why is there no word for "is" in Das stimmt?

In English, "right" is an adjective, so you need the verb "is" ("That is right"). But in German, stimmen is a verb that means "to be correct" or "to be right". So stimmt already contains the action. Adding ist (like Das ist stimmt) would be a grammatical mistake, as you would be incorrectly using two verbs.

Can I say Das ist richtig instead of Das stimmt?

Yes, Das ist richtig means exactly the same thing. Both are perfectly fine to use, but Das stimmt is a very popular, natural-sounding phrase in everyday German to agree with what someone just said.