Background and overview
In response to growing transportation demand in the Sapporo area during the mid-1980s, JR Hokkaido developed the 721 series suburban AC electric multiple unit (EMU), which entered service in 1988. The existing 711 series EMUs, equipped with partition walls between the passenger compartment and doorways to protect against severe cold, were causing chronic delays during peak hours due to prolonged boarding and alighting times.
The 721 series was designed to address these operational challenges by offering improved acceleration and deceleration performance, higher service speeds, and interior accommodations suitable for future services linking central Sapporo with New Chitose Airport, scheduled to open in 1992. A total of eight three-car sets—each consisting of two powered cars and one trailer car—were delivered between September and October 1988.
Car body
The carbody of the 721-0 series measures 20,800 mm in length and 2,900
mm in width, with a bogie centre distance of 14,400 mm. The structure is
primarily composed of lightweight stainless steel to reduce overall weight,
except for the bolster beams, the end sections of the centre beam, and
selected cross beams, which are fabricated from water-resistant atmospheric
corrosion-resistant steel plate. The exterior features unpainted bead-processed
stainless panels, with olive-green waistline and headline bands running
the full length of the car below and above the windows.
The cab end is constructed from weather-resistant steel and incorporates a gangway door and curved windscreens. A classification rollsign is mounted above the gangway door. Headlight and taillight clusters, enclosed in glass to maintain visibility in snowy conditions, are positioned above the windscreens, with additional headlights installed below. The cab end is painted silver, with an olive-green waistline band and black framing around the windscreens.
Each car is equipped with three 1,000 mm-wide single-leaf automatic stepped
sliding doors. Between the doors, two fixed windows and one drop window
are fitted. Door operation is controlled by TK103A equipment mounted above
the door headers. A roller-type destination indicator is installed on each
side of the carbody. Powered vehicles are fitted with snow separator louvers
employing a cyclone system to remove airborne snow from the intake used
for traction motor cooling. An N-AU721 air conditioning unit, rated at
20,000 kcal/h, is mounted at the centre of the roof.
Bogies
Motored bogies are designated N-DT721, while trailer bogies are N-TR721.
Both types feature bolster-less secondary air suspension, conical multilayer
rubber spring journal supports, and a Watts linkage bogie pivot. Conical
roller bearings and yaw dampers are employed to ensure stable ride quality
at speeds up to 130 km/h. Power transmission utilizes a parallel cardan
drive system with hollow shafts, and the gear ratio is 4.82. Both motored
and trailer bogies incorporate double-tread brake rigging with enhanced-adhesion
alloy cast iron brake shoes.
Traction unit and ancillary equipments
Each powered vehicle is equipped with eight shunt-wound brushed DC traction motors (N-MT721), each rated at 150 kW and featuring forced ventilation. These are controlled by the N-CS721 traction system, which employs thyristor phase control and field weakening to enhance high-speed performance.
Class Moha721 vehicles are fitted with a traction transformer (N-TM721) with secondary windings divided into four sections, a built-in smoothing reactor (N-IC721), and a traction rectifier (N-RS721) using a thyristor/diode hybrid bridge, all mounted beneath the floor. Class Kumoha721 vehicles are equipped with a roof-mounted traction resistor unit (N-MR721) for rheostatic braking, a motor-driven air compressor (MH-1045A-C2000), and an auxiliary power supply unit (N-APS721) rated at 8 kVA. These vehicles also feature a roof-mounted cross-arm type pantograph (N-PS721) and a vacuum circuit breaker (N-CB721). The braking system is an electric-command pneumatic type, utilizing rheostatic braking without regenerative capability.
Accommodation
Passenger compartments are separated from doorways by partition walls with
double-leaf sliding doors. Seating consists of 2+2 flip-over transverse
seats with a pitch of 910 mm in the centre section, and 1+1 seating at
the car ends. The original seat fabric is red, and strap hangers are installed
above the 1+1 seating areas. Class Kuha721 vehicles are equipped with a
Japanese-style toilet using a chemical holding tank, along with a water
tank. The driving cab features a one-handle master controller operated
with the left hand, marking the first such configuration among JR Group
companies.
Additional delivery
Vehicles manufactured from 1989 onward feature brown seat fabric. From September 1991, the casing of the traction resistor unit on class Kumoha721 vehicles was revised. In 1992, six-car train formations were introduced, prompting the development of new intermediate vehicles: class Moha720 (powered) and class Saha721 (trailer). Sets comprising Kumoha721, Moha721, and Saha721 are classified as the -100 subseries, while sets comprising Moha720, Moha721, and Kuha721 are designated the -200 subseries.
Class Saha721 vehicles feature a modified carbody in which the cab section of Kuha721 is replaced with a conductor's compartment. These cars also include a Japanese-style toilet with a chemical holding tank and water tank at the rear. To convert three-car set F-22 (built in 1991) into a six-car formation, trailer intermediate car Saha721-22 and powered intermediate cars Moha720-23 and Moha721-23 were additionally delivered.
Modifications
"U Seat" Reserved Seating Programme
Between November 2000 and January 2001, JR Hokkaido implemented a reserved
seating upgrade programme for 721 series vehicles operating on "Airport"
rapid services. The front passenger compartments of class Saha721 vehicles
and Kuha721-15, -17, and -19 were refurbished with 2+2 reclining seats
featuring a pitch of 1,030 mm, installed on a raised floor. These sections
were externally distinguished by red waistline and blue headline bands
running below and above the windows.
Current Collector Replacement
From 2004, the pantographs on class Moha721-0 vehicles were replaced with
single-arm type collectors (N-PS785) to prevent contact loss caused by
snow accumulation on overhead wires.
Side Window Replacement
Between 2002 and 2003, the side windowpanes were replaced with polycarbonate
panels to improve resistance against breakage.
Operations
The 721-0 series entered service in 1988 on "Airport Liner" rapid services between Sapporo and Tomakomai, "Marine Liner" rapid services between Sapporo and Otaru, and regional services on the Hakodate and Chitose Lines. In 1992, the "Airport Liner" and "Marine Liner" services were unified into the "Airport" rapid service linking Otaru and New Chitose Airport. These services were subsequently operated by newer 721 series sets capable of 130 km/h operation, while the 721-0 series was reassigned to regional services on the Hakodate, Chitose, and Sassho Lines alongside the 731 and 733 series.
Conversely, 721 series units operating on rural services between Iwamizawa and Asahikawa were replaced by the 737 series and began to be withdrawn from service in 2024. |
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