The K set is a class of electric multiple unit operated by Sydney Trains in Sydney & Newcastle, New South Wales, Australia. Like the L, R & S sets, the K set is a stainless steel double deck train. The "K" set introduced many new features to the suburban fleet prior to the introduction of the Tangara such as air conditioning and headlights. To accommodate the air conditioning and associated equipment, the pantograph had to be shifted to the adjacent trailer car to which the motor car is semi-permanently coupled and high voltage cables ran between the two cars. The motor cars have four four phase Mitsubishi 150 kW motors (as per R and S Sets), AARH\TightLock\Janney\Knuckle couplers and a maximum speed of 115 km/h. All carriages were constructed by A Goninan & Co.

There were two batches to the "K" set fleet, The first batch featured low mounted upper deck windows, brown interiors, and unpainted fronts. The second batch featured higher mounted upper deck windows, SRA "Candy" livery front, and yellow interiors, both of which have disappeared as a result of the "Citydecker" refubishment programme. Prior to the introduction of the Outer-Suburban Tangara "G" sets, some of the "K" sets from the second batch were used on the South Coast and Central Coast line (Via North Shore) peak hour services and these carriages were fitted with high back headrest seats.

Originally, the first four motor cars and driving trailers were the first to be fitted with air conditioning, later two more cars (C3550 and T4216) were also fitted with air conditioning to provide "spare" cars for the "air conditioned suburban train". All remaining "K" set cars were force ventilated until the late 1980s when the second batch (C3551/T4217–C3580/T4246) was fitted with air conditioning, this included the replacement of the "beclawat" windows with sealed non opening windows. The first batch was finally fitted with air conditioning just prior to the Sydney 2000 Olympics. These cars retained the "hopper" windows from its "Citydecker" refurbishment until the late 2000s, but they were sealed shut with an adhesive to avoid the loss of air conditioning. The vents on the doors were also boarded up.

There used to be some two car "K" sets (K1–K4), which were used on local Newcastle services, running between Newcastle and Gosford/Morisset. These cars returned to Hornsby for maintenance, which occured on a rotational basis. On such occasions, a four car "K" set was substituted on local Newcastle services. In special circumstances, two 2-car "K" sets were coupled together to form a 4-car set. In October 2013, K1-K4 were withdrawn from the Newcastle-Morisset shuttle services and formed into sets K98 and K99 in February-March 2014, with the former K98 being renumbered K91.

After a new timetable was introduced in November 2017, all K Sets were moved to Sector 2 and now operate on these lines:

  • T2
  • T3
  • T5
  • T6
  • T8

After the introduction of the new 2009 train time table, K sets were allocated to the Northern Line, where they ran in 8-car sets. This was due to the noise levels inside trains when operating on the Epping-Chatswood segment of the Northern Line; older R/S-sets do not provide enough sound insulation for passengers, while newer T/G sets do not have sufficient cooling in the regenerative braking system to cope with extended shuttle runs through the tunnel.

In July 2017, asbestos was found in the circuit breaker panels, which is inside the driver compartment, of the C and K sets. This caused all C and K sets to be taken out of service for inspection. As a result, S sets were used to replace C/K sets temporarily on services that were scheduled as C/K. All K Sets have since returned to service. In October 2017, K and C sets saw poles get painted yellow. The K sets are expected to stay in service until at least 2024, where they will likely be replaced by repurposed H sets after the latter gets succeeded by the Next-Gen Intercity Fleet.