Learn Spanish (Latin America): El hombre viaja en tren.

In English, we talk about traveling 'by' a mode of transport (by train, by car). In Spanish, the preposition en is used for this. You can think of it as traveling 'in' or 'on' the vehicle. This is the standard way to talk about transportation: en tren (by train), en avión (by plane), or en carro (by car).

Why isn't there an article here, like en el tren?

Just like in English where we say 'by train' and not 'by the train', Spanish usually drops the article (el or la) when talking generally about a mode of transportation with the preposition en.

Could I use por instead of en to translate 'by'?

No, when you are talking about the physical vehicle you use to travel, you must use en. While por sometimes translates to 'by' in other contexts, modes of transportation always use en.

How do we know viaja means 'travels' and not 'is traveling'?

Viaja is the present tense form of the verb viajar (to travel) for 'he', 'she', or 'it'. In Spanish, this simple present tense can translate to both 'travels' (a routine) and 'is traveling' (happening right now). Both translations are correct depending on the context!