Odakyu 3100 series NSE

The 3100 series sets were used on Hakone, Ashigara, and Sagami services.[5]

When the Odakyu 3000 series SE Romancecar and associated Hakone limited express began service in 1957, it proved to be very popular, and there were capacity shortages on weekends even with all 3000 series sets in use.[6] This would be exacerbated by the completion of the Hakone Ropeway in 1960 and tourism to Hakone increasing as a result.[7] Odakyu would introduce semi-express trains to the Odawara Line in 1959 using 2300 series and 2320 series trainsets,[8] but there were calls within the company to increase the number of limited express trains.[9] Ahead of the 1964 Tokyo Olympics, plans to increase the capacity of Romancecars were considered. Between building new 3000 series sets, lengthening existing ones, or manufacturing a new vehicle, Odakyu chose to make a new vehicle.[10] This was based on the idea that "the limited express vehicle is the icon of the company and must give passengers dreams with its novelty and splendor."[11]

Designs of multiple articulated trainsets connected together were considered, but ultimately a unified articulated 11-car trainset design was settled on.[12] The number of cars was chosen for concerns related to axle load and to match the 140-meter (460 ft) train length that the Odawara Line was being upgraded to in order to support 8-car express trains (Odakyu commuter train cars were 17.5 meters long at the time).[6]

While the 3000 series made a leap in technical performance compared to its peers when it was introduced, it was viewed as "somewhat poor" by Odakyu's sales department.[6] This was in part due to the 3000 series' focus on weight reduction, and launches of newer limited express trains by other railway operators that emphasized novelty and luxury. For this reason, the 3100 series would keep the technical advancements of the 3000 series, namely in its articulated design and low center of gravity, and focus its design around six slogans: Safety, Economy, Deluxe, Attractive, Comfortable, and Speed.[13]

Two trainsets would be tested between January and February 1963. On March 14, a 3100 series recorded a speed of 130 km/h during a test run, the highest speed recorded on an Odakyu line at the time.[14] Commercial operations began on March 16.[14] Two more trainsets would later be introduced, making for a total of four SE trainsets and four NSE trainsets.[15] The additional trainsets enabled Odakyu in a schedule update in November to adopt a 30 minute headway for Hakone services, and cut trip times between Shinjuku and Odawara down to 62 minutes.[11]

Three additional trainsets were added between 1966 and March 1967, enabling Hakone services to be run exclusively by NSE trainsets, except during train car inspections.[15] Odakyu chose seven trainsets under the presumption that a single trainset could make a round trip, including turnaround time, in 180 minutes, and that six trains offering service at 30-minute intervals would be sufficient. However, commuter demand would continue to increase beyond what Odakyu had forecasted, necessitating large scale construction at Shinjuku and a slowdown of limited express trains, increasing travel times between Shinjuku and Odawara to 69 minutes at fastest by 1972 after two schedule revisions. No new Romancecar trainsets would be introduced until the 7000 series LSE in 1980. OM-ATS equipment was added in 1968.[16]

Post-refurbishment 3100 series train in 1999

The trainsets would receive various refurbishments and updates between 1977 and 1987. Between 1977 and 1978, additional cooling equipment was added on the roofs of cars to improve cooling capacity and due to space limitations under the train.[17][18][19] The train interior would receive refurbishments between 1977 and 1980.[18] Electric locks would be added to train doors between 1978 and 1983 to prevent doors from opening while the train was running.[16] Following the introduction of the 7000 series LSE, the 3100 series sets received a larger scale repair and refurbishment between 1984 and 1987 by Nippon Sharyo.[20] This included additional interior updates, a new lighting arrangement, double glazed passenger windows, and a change of the front service sign from an acrylic plate to an electronic rollsign.[16][18] The capacity of the trainset was slightly lowered, with the capacity of cars 3 and 9 being reduced from 36 passengers to 32 in order to accommodate an expanded shop area.[16]

3100 series train in "Yume 70" livery in 1998

The series would begin to be phased out in 1996 with the introduction of 30000 series EXE sets.[21] In 1997, 3100 series set 3161 was turned into a special event limited express "Yume 70" and given a special livery in celebration of the 70th anniversary of operations on the Odawara Line.[22][23] Ahead of the July 1999 schedule revision, it was decided that the 3100 series would be discontinued and scrapped. Ahead of the retirement, Odakyu commemorated the retirement with 3,100 limited edition wristwatches and 3,100 limited edition Plarail units. An invitation-only farewell trip was held between Sagami-Ono and Karakida Station on July 11.[24] Regular Romancecar service concluded on July 16, 1999,[25] with a ceremony at Hakone-Yumoto, the first and only time a 3100 series trainset would depart from the station.[26] The mayor of Hakone gave a letter of appreciation at this ceremony.[26] After this, all 3100 series sets aside from the "Yume 70" set were taken out of service.[25] "Yume 70" would continue to be used as a charter train.[27] It performed its farewell run on April 23, 2000.[28]

• 
  Interior view, 2001

Initially, seven 3100 series cars were preserved. Six cars from the 7th trainset (lead car 3221) were held in Kitami inspection yard.[27] On October 2017, due to increased train storage requirements ahead of the completion of a quadruple tracking project on the Odawara Line, three intermediate cars from the set were to be dismantled.[29] The remaining three cars, two lead cars and one intermediate car, were relocated to the Romancecar Museum in Ebina, which opened on April 19, 2021.[30]

Lead car 3181, part of the 5th set, is preserved in a park near Kaisei Station.[27]

• 
  Preserved 3100 series car near Kaisei Station, September 2007

The 3100 Series is used as the basis for the Train in the Vary Dimension in the series finale of the 1966 TV series Ultra Q.