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Train OperationsⅤ
High Speed Multiple Units
The modern two-locomotive concept for push-pull operation first appeared in 1959 with the UK’s Blue Pullman series of trains.A diesel power car was provided at each end of a six（or eight）coach set.The concept was further developed in the 1970s with the UK High Speed Train（known as the HST）and in France with the TGV（Train à Grande Vitesse）.The former is diesel powered，the latter electric but the concept is the same.Both these trains use a power unit at each end with a set of passenger carrying coaches in-between.The Germans have joined the club with their ICE train.The only real difference between these trains and the original push-pull concept is that the newer trains were purpose built.
Not forgetting the Japanese high speed train concept；they were the first to introduce over 200km/h running on a regular basis and have kept at the forefront of high speed train technology with their German and French counterparts.Nevertheless，the Japanese HSTs have always been multiple units in the original sense，having many power cars distributed along the train.
The HST name was first used for diesel multiple unit passenger train developed in the UK for 125 mile/h running.It is now usually accepted as the definition for any passenger train scheduled to run at over 200km/h.
Headway
This is the name given to the elapsed time between trains passing a fixed point in the same direction over the same track.It is generally expressed inminutes e.g.“trains were running at a 4-minute headway”.Another way of expressing it is as trains per hour（tph）.
A well run railway will conduct research to determine how many fare paying customers are probably to show up at various times of the day and will operate their trains to suit.In many cases the patronage numbers will show that it is possible to run trains at even intervals or at a given“headway”.This may be at two hours for a long distance，main line route or twominutes for a metro.
Once established，the headway is used in calculating the number of trains needed for a particular service，the train performance requirements and signalling requirements.