The 400 series is the first generation of the mini-Shinkansen, a high speed train that can both run on the high speed rail and normal train lines. The 400 series can put to the top speed of 240 km/h on the high speed rail, and on a lower speed when running on the normal rail. Since the normal trains are running on the narrow gauge railway instead of the standard gauge railway which the Shinkansen are using, engineers need to build an extra rail on the original track so that it can both fit for the mini-Shinkansen and normal trains. The train also has a smaller cross-section body to fit the standard train size that designed for normal trains, and use pedals to fill the platform gap when the train is on the high speed rail station.
Why do you think the engineer would design this mini-Shinkansen instead of building a new high speed rail?
Jasper's Blog on Trains 