The Mark 2 has a semi-integral construction, giving it more strength than a Mark 1 in the event of an accident. A key driver of the changed construction method was to overcome the serious corrosion problem point in the Mark 1 at the base of the body, where it was attached to the underframe. Other changes of design, such as the window units, were for the same reason, which had become a serious problem in Mark 1 vehicle maintenance costs. Revised painting methods were also part of this, which coincided with the change of livery from maroon (dark green on the Southern) to the blue and grey that Mark 2 coaches wore for much of their lives (some of the earliest Mark 2 coaches had the old livery at first).