During this period, a number of prototypes were constructed to explore different designs and approaches for implementing the railbus concept. One such vehicle was a single two-car unit, designated Class 140, built between 1979 and 1981. This prototype was introduced with much fanfare in June 1981. Initial testing with the Class 140 uncovered several problems, such as difficulty detecting the type via track circuits. This was reliably resolved by changing the material of the brake blocks from a composite to iron. Two less easily addressable drawbacks were the high level of noise generated during transit, particularly on older jointed rails, a consequence of the railbus's direct connection between the underframe and suspension with the body that transmitted impact forces across the body. It was also observed that the inclusion of strengthening members in the mass-produced bus body added significantly to the overall production cost, which eliminated much of the cost advantage that was the primary goal of the type.