Learn Spanish (Spain): ¿Viajaréis en tren? - Will you guys travel by train?

In Spanish, the simple future tense is formed by adding endings directly to the full infinitive of the verb, rather than taking the -ar off first. For the vosotros (you all, informal) form used in Spain, the ending is -éis, making viajaréis (you all will travel).

Why do we use en here instead of por to mean by train?

When talking about means of transportation in Spanish, you almost always use the preposition en. While in English we say by train or by car, in Spanish you use en, as in en tren or en coche.

Do I need to include the word vosotros before viajaréis?

No, you do not need it. Because the -éis ending is only ever used for vosotros, it is already completely clear who is doing the action. You can add it for emphasis (¿Vosotros viajaréis en tren?), but it is usually left out.

I learned ustedes for you guys. Can I say ¿Ustedes viajarán en tren? instead?

Yes, you can! Ustedes viajarán is perfectly understood everywhere. However, in Spain, people use vosotros and its specific verb endings for informal groups like friends or family, saving ustedes for formal situations. In most of Latin America, ustedes is used for both.