sentences:  Chiltern Railways has obtained four two-car trains in the same order as that of London Overground to supplement the current Class 165 "Networker Turbos" on its routes from London Marylebone, originally due to be delivered from late 2010.They do not have the tripcock safety system installed—unlike the rest of the Chiltern Railways fleet—as the design of the bogie being used (based on the B5005 found on Class 220 "Voyager" and Class 222 "Meridian" trains) means there is no place for the brackets to be mounted.Thus, they are not able to operate on the Aylesbury via Amersham line whilst the current signalling system is in place unless running in multiple with other units on both the front and rear ends, which are fitted with a tripcock.