I think when a passenger has a pre-existing medical condition which may preclude alternative forms of transport as described by the OP, the onus in the first instance lies with the passenger. Most delays that require replacement transport are either pre-planned engineering works, which can easily be planned around, or inclement weather conditions which themselves can be forecast at least a few hours / couple of days in the future. So where either exists, the passenger should either re-plan their journey via an alternative route from their starting point, or consider cancelling.
When blockages occur without any pre-warning, a passenger requiring transit via rail should make representation to a staff member of their TOC as soon as it becomes apparent that a problem may exist. Calmly explaining the situation as soon as possible gives them the most opportunity to explore possible alternative routes. However depending on the routes, time of day, passenger flows it may still be the case that an alternative TOC refuses to accept a ticket without at least an excess being paid, which the passenger should be aware of from the outset. Even if viable routes exist, it may be that earlier flows of passengers as a result of the blockade mean that the alternative TOC will be unwilling to accept further passenger numbers due to anticipated over-crowding. Any passenger aware that they may not be able to use alternative forms of transport need to be aware of these possibilities, and be prepared either to deal with the routes offered, or abandon the journey and return to their starting point (which will also need discussion with the TOC).
This might all sound heartless, but disruptions do happen and the individual needs of every passenger may not be able to be accommodated every time, and TOCs simply cannot be expected to have contingency plans for every type of medical condition that their passengers may suffer from.