What was the real world (UK) practice? Did all single track lines use tokens? Was absolute block absolutely limited to double tracked lines?
Yes - in the UK, single track lines always (so far as I am aware - very early railways might have differed, and self contained networks with only one train in steam at any one time would of course have had no signalling at all) had to have some additional protection for single track sections. In the very early days, a staff was used: only the train that had the staff may enter the section, and there was only one staff. It was quickly realised that this was inadequate, as this only allowed for trains travelling in alternating directions. Thus, the staff and ticket system was developed, allowing one train to follow another: the idea was that the driver of the first of two trains in the same direction would inspect the staff at the start of the single track section, and receive a ticket, but leave the staff at the start of the section for the next train. This staff and ticket system worked, but could give rise to difficulties in cases where things did not work as expected, and it was not uncommon for a deadlock to arise with the result that it was necessary to send somebody on horseback to fetch the staff from the other end of the section.
In, if I recall correctly, the 1860s a more sophisticated system was invented, the token block system. This was an electrical equivalent of the staff and ticket system: there would be a stack of tokens at token machines at each end of the section. A train would have to have one token to enter the single line section. However, only one token could be withdrawn from the machines at either end of the line at a time, although multiple tokens could be stacked on top of each other at either machine to allow a number of trains to pass in the same direction. There were some variations on this basic idea, but this system remained dominant on single track lines until track circuits were introduced which eventually made this additional layer of safety redundant (although not immediately).
There has been intermittent discussion about trying to model the staff and ticket system in Simutrans-Extended, but this is not workable because either it is very awkward for players to use and (realistically) prone to deadlocks (which would be unmanageable in online games) or it would be functionally identical to token block.
