sentences:  The Class 172 trains are lighter than other Turbostars due to use of the Bombardier FLEXX-ECO bogies - a development of the B5000 bogies used on the Class 220 "Voyager" DEMU trains, rather than the previous 'Series 3' bogies.Another difference is that they have half-height airdams as opposed to the more standard full height airdams.They also differ from earlier Turbostars in having mechanical transmission rather than hydraulic - gear changes can be distinctly heard as the trains accelerate and decelerate.