Ash-Shaykh Saalih al-Uthaymeen, may Allah have mercy upon him, was asked on the ruling of a person boarding a bus which contains of
free-mixing between men and women?

A: “It is compulsory upon the person to keep far from touching women, I
mean from crowding them, whereas his body becomes connected to her body even if (that were to occur) behind a screen, for indeed this calls to fitnah. The human being is not safe (from everything); he might see himself that he is cautious of this matter and that he will not become affected by it however ash-Shaytaan flows in the blood stream of the son of Adam, so perhaps a (slight) movement from him may corrupt him. So if it is a must for the person to do so (to aboard on a free-mixing bus) while him striving not to become affected then I hope there is no harm in it however my opinion is that it is not
possible to be a must for him to do so, due to it being possible for
him to seek a place where he will not be connected to a woman even if he has to stand (throughout the ride) and by this he frees himself
from this affair that which necessitates fitnah, trail and it is upon the person to fear Allah according to his capability and not to see these affairs to be minor. Likewise as we hope from those who are in charge of these means of transportation; whether it be any means of transportation to make a place specifically for women whereas they are
not connected to the men.”

Reference: Fataawaa Islaamiyyah (3/119).