Learn Farsi: آنها شاید قطار را لغو کنند.

No, shâyad is an impersonal modal verb. That means it never changes its form or takes personal endings. It is always just shâyad, whether you are saying "I might", "they might", or "we might". All the conjugation happens on the main verb at the end of the sentence (konand).

Why is the verb laghv konand instead of laghv mikonand?

The word shâyad requires the main verb to be in the present subjunctive form to show uncertainty. For compound verbs that use kardan (to do), you drop the mi- prefix to make it subjunctive. So, the indicative mikonand (they do) becomes the subjunctive konand.

I thought the present subjunctive needed a be- prefix. Why isn't it laghv bekonand?

While the be- prefix is standard for the subjunctive of most simple verbs, it is usually dropped for the verb kardan when it is part of a compound verb like laghv kardan (to cancel). You might occasionally hear bekonand in spoken Farsi, but konand is the most common and standard form.

What is the purpose of râ after qatâr?

râ is a marker that shows the definite direct object of a sentence. Because they are canceling the specific train rather than just any general train, we must use right after (train) to point out exactly what is receiving the action.