Prepositions: When indicating manner, are *a* and *en* interchangable?

Question

The way it is presented in the grammar topic about prepositions in Beta, as shown in the quoted part below, I get this impression. Is this true in all such cases or only, as one could make up from the examples, only when it concerns a mode of transportation?

A

indicate manner, how something is done (on, by, with)

•Vamos a pie. (We went on foot.)

•Lo hice a mano. (I did it by hand.)

•Escribe a lápiz. (He writes with a pencil.)

En

to indicate manner, how something is done (by)

•Vamos a Italia en tren. (We go to Italia by train.)

•Me voy al doctor en coche. (I go to the doctor by car.)

Answer 1

I don't think they are interchangeable, although there are probably some situations where they are. You can say

Vamos a pie

but not

Vamos en pie. Although I think vamos de pie does work.

I don't think you can switch a for en in any of your examples.

Personally, I would probably go with *por* in each of those 5 occasions. It just confused me the way it was presented. - S1r_Wakka, Feb 2, 2012

Answer 2

I think you say Vamos por tren, por avion etc when it's a manner of going. but de pie like the last guy said