The trainsets of Class VT 11.5 (often simply called TEE) were diesel multiple unit

Diesel multiple unit

A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar or railmotor, depending on country.-Design:...

s built by Deutsche Bundesbahn

Deutsche Bundesbahn

The Deutsche Bundesbahn or DB was formed as the state railway of the newly established Federal Republic of Germany on September 7, 1949 as a successor of the Deutsche Reichsbahn-Gesellschaft '...

 in the 1950s and used for Trans Europ Express

Trans Europ Express

The Trans Europ Express, or Trans-Europe Express, is a former international railway service in Europe. At its maximum extent, in 1974, the network comprised 45 trains, connecting 130 different cities.-Origin:...

 services. When the new numbering scheme of the German railways became effective on January 1, 1968, the power heads of the trainsets were renamed to Class 601 whilst the middle cars became Class 901.

The first trains were commissioned in 1957 and used on the following international TEE lines:

• TEE 31/32 Rhein-Main Frankfurt/Main – Amsterdam
• TEE 74/75 Saphir Dortmund – Oostende
• TEE 77/78 Helvetia Hamburg Altona – Zürich
• TEE 168/185 Paris-Ruhr Dortmund – Paris Nord

In the 1960s, due to ongoing electrification

Electrification

Electrification originally referred to the build out of the electrical generating and distribution systems which occurred in the United States, England and other countries from the mid 1880's until around 1940 and is in progress in developing countries. This also included the change over from line...

, lines like the TEE Helvetia were changed to electric locomotive-hauled train sets.