Indeed.
I think that as far as messaging is concerned, a simple:
"We have limited capacity, please consider alternative means of transport or try to travel at quieter times if you can" would suffice and fit with the guidance.
Sounds sensible and logical. Which is why it probably won't happen, for months anyway.
Next logical step would be "You are welcome to travel with us for any reason, and you must wear a face covering. However, we cannot guarantee social distancing at all times, and please respect social distancing where it is possible".