Learn Spanish (Latin America): Son las dos y los caballos frenéticos rompen a galopar. - It is two o'clock and the frantic horses break into a gallop.

The phrase romper a followed by an infinitive expresses the sudden beginning of an action. While empiezan a simply means they start to, rompen a emphasizes that the action started very abruptly and with intense energy. This perfectly captures the English phrase break into a gallop.

Since this lesson also covers echarse a, could I say los caballos se echan a galopar instead?

Yes, you absolutely could. Both romper a and echarse a express a sudden start to an action. The phrase echarse a is extremely common with verbs of motion like running or flying. However, romper a carries a slightly more explosive or disruptive nuance, which fits nicely with the fact that these horses are frenéticos.

Why doesn't the sentence say rompen a galopando since the action is ongoing once it starts?

In Spanish, prepositions like the a in romper a are almost always followed by an infinitive, never a gerund like galopando. Even though the galloping continues after it begins, the grammatical rule dictates using the infinitive galopar.

I always second-guess myself on telling time. Why is it son las dos and not es las dos?