English divides time up differently from French. It might help you to understand the subtleties in English first -- most native speakers haven't ever had to think about it.
In English, we (subconsciously!) divide verbs into active and stative verbs. Roughly, the distinction is that an active verb is something you do whereas a stative verb is something you are or experience. Active verbs include go, walk, hit and stative verbs include be, love, miss.
In English, for an active verb, we use the present tense if we wish to talk about habitual action, and we use the present progressive if we wish to talk about current action. For instance, consider the difference in the sentences "I am going to work. I go to work."
For a stative verb, the present tense talks about action right now, and the present progressive is rare, emphasisizing something particularly transient about the action. "I miss you." "I see a fruit on the table." Non-native speakers often bungle this, saying things like "Are you smelling coffee?"
However, if we really want to insist that the stative action is happening right now, we can indeed use the present progressive, even though it's rare: "I'm lovin' it" [MacDonald's slogan]. "I'm reading your favorite book right now [usual use of progressive for active verb read] and I'm missing you [unusual use of progressive for stative verb miss, converts "miss" to an active sensation instead of the usual use]." You can think of this as converting to an active verb.
What has this got to do with French? The simple answer is that French does not show this distinction between active and stative verbs in the present tense. The simple present can refer to habitual action or to action that is taking place right now, whether the verb is active or stative.
Je vais à l'école.
can mean either that I go to school every day, or that I'm going right now. There is no need to make a distinction unless it's important to the conversation.
So if you want to insist that the action is right now, and not habitual, you need some other expression; this is where « en train » comes in.
Je suis en train d'aller à l'école, est-ce que je peux vous rappeler plus tard ?
Note that native English speakers tend to overuse "en train" with active verbs, when we translate English word-for-word. This sounds funny to them the same way "I am smelling coffee" [stative verb wrongly in progressive] sounds a little funny to us. In French, you only use it when it's necessary to the conversation to indicate that the action is right now and not habitual.