Should the government and other agencies arrange everything in a democracy? You can also appeal to your citizenship to keep the necessary distance between others.
If it's too busy on that train and you feel you can't keep the social distance apart, get off and take the next less crowded train just like you would on a bus. These transport companies already provide extra trains and buses, only to allow you as a traveler to keep your distance.
In an airplane it is different you can not compare with the above, here are the airlines that must ensure that you can always keep social distance from a fellow passenger, when boarding and disembarking, but also during the flight.
This will take a lot of inspiration from the airlines, but I am sure that a solution can be found to the satisfaction of those who want to take the plane. The solution has to come, because it is now a matter of surviving for the airlines, they absolutely cannot turn their heads or cry as little babies at the ice cream shop.
It's just a new world and everyone will have to put in an extra effort for a while before it can return to normal.