The Class 373 Eurostar is an electric multiple unit that is used for Eurostar international high-speed rail services from the United Kingdom to France and Belgium via the Channel Tunnel. They were built between 1992 and 1996 and were constructed at four different locations in the UK, France and Belgium.

They are part of the TGV family of trains but built with a smaller cross-section to fit the smaller loading gauge in Britain.

The Class 373s were originally constructed as a dual voltage units capable of operating on the UK third rail network and on the AC overhead wires. When the first services commenced to the continent via the Channel Tunnel in 1994 they ran to and from London Waterloo via the existing third rail network. Four new platforms were constructed as well as an arched station roof which formed the new Waterloo International Station. After the opening of the Channel Tunnel Rail Link, later renamed High Speed One, services moved to the refurbished St Pancras International station.

There were two types of Class 373s constructed.

These comprised of thirty one Three Capitals sets consisting of two power cars and 18 passenger coaches, measuring 387 metres in length. The length of a complete set is dictated by the Channel Tunnel safety regulations as the distance between consecutive cross passages is 375 metres. This means that if a Eurostar train has to stop inside the Tunnel in case of an emergency, it would always stop adjacent to a cross passage.

There were also seven North of London sets also known as "Regional" Eurostars with 14 coaches and two power cars, which were just over 312 metres in length. These sets were intended to provide Regional Eurostar services from continental Europe to and from north of London, using the West Coast Main Line and the East Coast Main Line, but these services never came to fruition for a variety of reasons.

In May 2000, two Regional Eurostar sets were leased to GNER to operate The White Rose services from London King's Cross to York. From May 2002, the White Rose was altered to operate to Leeds with a third set leased. Some of the sets had the GNER livery applied, whereas the rest carried the original Eurostar livery without logos. The lease expired in December 2005 and they were handed back to Eurostar; they were later used to operate high speed TGV services with SNCF in northern France.

SNCF leased 3 of Eurostar's "Three Capitals" sets for use on French domestic TGV services mainly between Paris and Lille.

The sets remained in the original Eurostar livery with SNCF branding, and some sets had greyish white or silver front ends. In 2007, SNCF added more Class 373 sets to its fleet by leasing the redundant "North of London" sets from Eurostar.

In October 2014, the three "Three Capitals" sets were withdrawn from traffic and stored, having been replaced by TGV Duplex sets. Some have since been scrapped having provided spare parts to other Class 373 sets with remaining sets still stored France.

On the 30th July 2003 a Class 373 Eurostar set a new British rail speed record when it reached a speed of 334.7 kilometres per hour or 208 miles per hour on the approach to the Medway Viaduct.

In 2008 the UK based Class 373s began to receive a mid-life update which included a redesigned interior and a new livery.

In October 2010 Eurostar ordered 10 Class 374 Velaro trains from Siemens. The intention was to retain 8 Class 373 sets to run alongside the new trains, the rest of the fleet would either be scrapped or stored.

The recent pandemic has seen a dramatic reduction in international travel which has meant far fewer Eurostar services running across to France. The few that have run have been operated by the newer Class 374s. However as things are now starting to return to normal a Class 373 has returned into service in recent weeks on test/ training runs along HS1.