An initial order was placed in July 1997 for 120 vehicles to replace the Connex South Eastern class 411 fleet followed by a further order for 90 vehicles to replace CSE's small fleet of class 421s. These were funded by HSBC Rail. A further order was then placed for both Connex's SE and South Central services. However, with the loss by Connex of the South Central franchise from May 2003 and the takeover of South Central services by Govia South Central from the 26th August 2001 the South Central portion of the order was adopted by Govia - these and subsequent South Central orders will be detailed later. Subsequently Connex was removed by the SRA from the SE franchise which is now being run by South Eastern Trains. The total 112 unit fleet for South Eastern services was as follows:

375/3 3 car express dc only units, formed DMSO-TSO-DMSO - 10 units
375/6 4 car express dual voltage units, formed DMSO-MSO-PTSO-DMSO - 30 units
375/7 4 car express dc only units, formed DMSO-MSO-TSO-DMSO - 15 units
375/8 4 car express dc only units with modified shoegear, formed DMSO-MSO-TSO-DMSO - 30 units
375/9 4 car outer suburban dc only units, formed DMSO-MSO-TSO-DMSO - 27 units

All dc only units are capable of becoming dual voltage with the installation of extra equipment, the rolling stock provision now being devised with the possibility of leasing units to other TOCs in the future in mind. The construction of the cars is aluminium with steel ends. Multiple operation up to twelve cars will eventually be possible. Tightlock couplers were originally fitted to /3, /6 and /7 batches and Dellners to /8 and /9 but it is believed that the Tightlock fitted units will be retro-fitted with Dellners. Unlike South Central units this will not result in a change to the unit class number. Four car sets have six axles motored, each axle with a 250kW traction motor controlled by an independent inverter drive. The units' regenerative braking will not be used except in its rheostatic mode until the power supply system is proven to be capable of accepting the additional intermittent power input. Bogies are of the Bombardier P3-25 (power) and T3-25 (unpowered) designs.

The first unit departed Adtranz's Derby works in April 1999 to Chart Leacon for testing and mileage accumulation as part of the lengthy Railtrack Safety Case procedure. Construction of the remainder of the order began in January 2000. The cars are supplied in overall white livery, Connex/South Eastern Trains colours and brandings being applied with vinyl overlays. Maintenance of the SET 375 fleet is at Chart Leacon as a SET/Bombardier joint venture. Introduction to service of the class has been slow whilst de-bugging takes place and the contractual reliability targets achieved, however full introduction to service which would allow major service improvements is impeded until the power supply systems are upgraded. There were initial difficulties with coupling and uncoupling en route which resulted in services being recast to avoid the need for this operation. A further problem occurred in January 2004 due to problems with the cab doors and this resulted in a temporary withdrawal of all SET Electrostar units.

A possible future configuration which was being considered was a five car variant by adding another MS to a four car formation. In August 2003 this was trialled with a temporary reformation of unit N°375604 containing additional car 74253 (from unit N°375603) marshalled next to motor coach 67804. 5 car unit testing was completed in September of that year. All SET Electrostars had selective door opening enabled in August 2003.

In 2004 375s were fitted with dedicated First Class sections in the twelve seats behind each driving cab. This meant that on a twelve car train formed from four car units there are six sections of First Class, four of which have to be open so people can move through the train.

The first SC Electrostar was delivered to Brighton in February 2002 for testing and by April of that year six sets were commissioned and available at Brighton for driver training. These early units were delivered in white primer whilst the remaining were delivered in the new green and white livery that was from the Summer 2004 timetable re-branded as "Southern Railway". The SC units will be maintained, again as a joint arrangement with Bombardier, at Selhurst. In terms of traction motors and brakes they are identical to SET units. One major difference between SET and SC units is that SC units have external TV cameras so the driver can monitor door operation without leaning out of the cab door window.

The early 3 car 375s were used on Coastway diagrams, in particular 313-322 were dedicated to this as they are standard class only. However the 2+2 seating is much appreciated by regular travellers on this route. From late March 2003 fast London-Brighton services went over to twelve car formations of 3 car 375s, particularly using 323-338 which have "First" accommodation. From the autumn of that year most Brighton main line services were being worked by 375s due to difficulty manoeuvring a catering trolley in3+2 pattern 377s. The Electrostars have not necessarily proved popular with South Central travellers who do not find them particularly comfortable. Their interiors are narrower than the Mk1 EMUs they replace and this is aggravated where there is 3+2 seating. The hunt for the relatively more comfortable wide 2+2 seating in parts of the 377/1 units has turned some regular travellers, who were not already so, into train spotters. The formula is follows:
On units 377101-119 there is 2+2 seating throughout.
On units 377120-140 the 2+2 seating is in the outer end portions of each car with 3+2 seating in the centre portions of each car.
On units 377141-164 the 2+2 seating is in the driving cars. There is also 2+2 seating in the outer ends of the intermediate cars, which are otherwise 3+2, however this uses seats as narrow as those in the 3+2 section (albeit with a slight spacer in between effectively increasing the width a fraction). Note that leg spacing is slightly more generous in the outer portions of each car than in the central sections. It appears this latter formula also follows for the dual voltage (377201-215) units.

Selective door opening tests on South Central commenced on 24th March 2003. Test trips using dual voltage units between Victoria, Kensington Olympia and North Pole Junction. commenced in November 2003 followed by some workings between Euston and Watford in February 2004.