|Number built:||63 (21 3-car units)|
|Engine:||Perkins 2006TWH diesel per car|
|Power:||1, 050 hp (780 kW)|
|Formation:||Driving Motor Composite Lavatory (DMCL)+Motor Standard Open (MSO)+
Driving Motor Standard Lavatory (DMSL)
Originally operated by Network South East all 166s are now operated by GWR (though many carry the earlier First Great Western branding still).
Originally both driving cars had first class accommodation but in 2014 one DMCL was declassified in order to increase the overall capacity .
|GWR 166 218 at Reading|
|GWR 166 209 at Worcester Shrub Hill|
|GWR 166 212 at Worcester Foregate Street|
|GWR 166 221 alongside HST 43 322 at Paddington|
|GWR 166 218 at Guildford|
 Colin J. Marsden (ed.), "Class 166 'Networker Express'", Modern Locomotives Illustrated No. 216 December 2015-January 2016 (Second Generation DMUs Classes 165-185). p. 14