The more or less literal translation “j’allais” works. There is no subjunctive involved.

J’allais partir quand le téléphone a sonné.   (I was about to leave when the phone rang.)
J’allais le faire hier mais j’ai dû traiter une demande urgente à la place.   (I was going to do it yesterday but I had to process an urgent request instead.)

“J’étais en train de” means that you were already doing the task.

“J’allais” is generally good but there can be other translations. For example, “I was about to …” may be translated as “Je m’apprêtais à …” to indicate imminence, i.e. that you were just about to start (as opposed to merely planning to do it later). Conversely, to indicate that you were planning something but hadn’t committed to starting, you can say something like “J’avais prévu de …” (“I had planned to …”).