>>1653785>Subways are also incompatible with national railways and in subway-dominated cities commuter rail to suburbs is, in general, bad.The main reason why subways/metros are isolated systems from national railways is for capacity and also, typically other reasons relating to the more efficient use of rail in a metro system compared to a heavy rail mainline. Signal blocks, for example, are much shorter in metro systems to increase capacity - the trains are capable of accellerating and decellerating quickly enough for this to work. Heavy rail trains cannot do this and need the bigger signal blocks to respond promptly.
That said, there typically isn't any incompatibility between metros and mainlines in any meaningful sense outside of the loading gauges which tunnels may be built to. Standard gauge is almost always used (except in Spain, where there's some incompatibility between Standard and Iberian Gauge railways), and the only real "incompatibility" comes from electrification, which can be "solved". Either a decision is made to use overhead electrification, or a decision is make to use 3rd rail. The London Underground has 4th rail electrification for reasons...
But quite often, these systems will have railway junctions which can connect mainlines to the subways for a variety of reasons, but not for regular service use.
The London Underground has a few sections where both National Rail and the Underground share rail, though this is limited. And there are ways for trains to go from NL onto Underground rail for a variety of reasons, such as bringing maintenance equipment and locomotives, delivering rolling stock or even bringing special trains on and off of the Underground network.
