 These trains also had automatic sliding doors (as opposed to the manually-operated doors on the S-series trains), lightweight car bodies (10% lighter than the S units), and coil-spring rather than leaf-spring suspension.They were used extensively on the suburban lines out of Moscow and St. Petersburg, where they significantly reduced travel times; however, their biggest shortcoming was that their entrance/exit doors were only designed to be compatible with high platforms, which precluded their use on other lines (most of which had stations with low platforms).As stated in the previous section, there were many busy suburban railroads in the Soviet Union which required a faster and more powerful replacement for the obsolete S units, but also one which was compatible with the low passenger platforms used on these lines (which the ER1 could not accommodate).