Context: On tracks that London Underground share with National Rail third-rail stock (the Bakerloo and District lines both have such sections), the centre rail is connected to the running rails, allowing both types of train to operate, at a compromise voltage of 660 V. Underground trains pass from one section to the other at speed; lineside electrical connections and resistances separate the two types of supply. These routes were originally solely electrified on the four-rail system by the LNWR before National Rail trains were rewired to their standard three-rail system to simplify rolling stock use.

Question: What is the voltage shared two types of trains UK railroad system?
Answer: 660 V

Question: Why some sections of Bakerloo and District lines were rewired to three-rail system?
Answer: to simplify rolling stock use

Question: How did it become possible to share the voltage for different types of train?
Answer: the centre rail is connected to the running rails

Question: What joins the two types of supply?
Answer: Unanswerable

Question: What was rewired to the standard four-rail system?
Answer: Unanswerable

Question: What is the maximum voltage of the two UK rail systems?
Answer: Unanswerable

Question: Above ground trains pass from one section to the other at what?
Answer: Unanswerable

Question: The London Underground does not share tracks with whom?
Answer: Unanswerable