In a railway accident, telescoping occurs when the underframe

Underframe

An underframe is a framework of wood or metal carrying the main body structure of a vehicle....

 of one vehicle overrides that of another, and smashes through the second vehicle's body. The term is derived from the resulting appearance of the two vehicle bodies: the body of one vehicle may appear to be slid inside the other like the tubes of a collapsible telescope - the body sides, roof and underframe of the latter vehicle being forced apart from each other.

Telescoping often results in heavy fatalities if the cars telescoped are fully occupied. The car riding on top will often be destroyed by the structure of the car below (physics of the incident can often reverse this damage), leaving very little survivable space. The chances of telescoping can be reduced by use of anticlimbers and crash energy management (CEM) structural systems.

Accidents where telescoping occurred are numerous and include:

• The Illinois Central Gulf commuter rail crash in Chicago
• Seer Green rail crash
  Seer Green rail crash
  The Seer Green rail crash occurred on the morning of 11 December 1981 near Seer Green, Buckinghamshire, England between two four-car Class 115 units killing one driver and three passengers.-Events:...
  
  
• Moorgate tube crash
  Moorgate tube crash
  The Moorgate tube crash was a railway disaster on the London Underground, which occurred on 28 February 1975 at 08.46 am.A southbound train on the Northern Line crashed into the tunnel end beyond the platform at Moorgate station...
  
  
• Lewisham rail crash
  Lewisham rail crash
  The Lewisham rail crash on the British railway system occurred on 4 December 1957 just outside St Johns railway station in Lewisham, south London...
  
  , 1957
• 2008 Chatsworth train collision
  2008 Chatsworth train collision
  The Chatsworth train collision occurred at 16:22 PDT on Friday September 12, 2008, when a Union Pacific freight train and a Metrolink commuter train collided head-on in the Chatsworth district of Los Angeles, California, in the United States...
  
  
• June 22, 2009 Washington Metro train collision

To reduce the chance of telescoping, rail and tramway vehicles are often provided with an anticlimber: a horizontally ridged plate at the end of the chassis, which in a collision will engage with the anticlimber on the next car.