&gt;  For one, it&#x27;s common sense that a train that disappears from the system isn&#x27;t really gone-- but the Metro system couldn&#x27;t take that into account. Stuff like that should be pretty trivial today.<p>It was pretty trivial in the 1970s. Require the sensor for an adjacent segment to be engaged before the sensor for an active segment will disengage.<p>You can&#x27;t do any computation with modern computers that you couldn&#x27;t have done with relays in the 1970s, or the 1870s for that matter. Mechanical relays are slower, less reliable and more expensive than transistors... but they do the same thing.