French has one present tense only. English has two (present simple and present continuous). Depending on context, the English present and present continuous always translate to the French present tense. There are some exceptions where one might use en train de, but there is no hard and fixed rule for this.
I speak French every Tuesday at school = Je parle français tous les mardis à l'école.
I'm working at IBM this week = Je travaille chez IBM cette semaine.
That said, in French, être en train de would usually translate as the present continuous in English.
Où sont-ils? Réponse: Ils sont en train de faire la lessive. Translation: They're doing the laundry.
In the example given, I'm taking an exam. The normal translation is: Je passe un examen. Only in some circumstances might one say: Je suis en train de passer un examen. [Presumably, if you are taking an exam, you would be able to talk about it to anyone unless you're on a cell, which is probably not allowed]. But even in this case, in French, Je passe un examen would be understood by a French speaker as something happening in the present continuous sense of the verb (am taking) in English.