Does my ablution become void by sleeping on public transport if I suspect it was a deep sleep?

Ablution is not nullified except by certainty of the occurrence of that which nullifies it. As for mere doubt, it does not nullify ablution, even if it is a strong doubt. The proof for this is the hadith: "(He should not leave) until he hears a sound or finds a smell," and Imam An-Nawawi's statement that this hadith is a fundamental principle that things are judged to remain on their original state until there is certainty to the contrary. Whoever is certain of purity and doubts impurity is judged to remain in purity. There is no difference between the doubt occurring inside or outside of prayer. This is the view of the majority of scholars. Therefore, if a Muslim doubts whether his sleep was deep or not, his ablution is not nullified, because purity is established with certainty and is not removed by doubt.