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>[Dahl and Nygaard] had a concept of timing which was implemented as a simulated time train in the way that is completely standard nowadays. That gives a framework within which one could explore the meaning of real concurrency.

Could anyone comment on what that framework is in current terms? Googling 'simulated time train' doesn't really help. Thanks!

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Not sure what he means by "train", but Simula 67 included a module for performing "simulations", which was the original point of the language (Simula 1 was a simulation-specific language; Simula 67 was a general-purpose language with simulation in the standard library.)

The simulation module was a framework for defining parallel processes that worked with discrete, scheduled events, a bit like actors. This was intended to model real-world problems such as traffic systems or factories, that complex interactions and dependencies. A simulation had a time axis, so it ran the processes in simulated time, instead of real time. This may be what Hoare was referring to.