<p>The rear wheels (20) are driven by electric motors (15) and gearing (17) from a power unit (21). A three-phase field winding of each motor is switchable between star and delta connections by a mechanical switch (25) in its power train (23) under electronic control (27). The accelerometer (31) fitted to a part (29) of the structure is wired to a processor (33) which responds to the amplitude and/or rate of change of its output. The tractive force is reduced in accordance with a stored characteristic curve (37) when the acceleration rises to a dangerous level.</p>
