Development of Locomotive 25 



express engines. This was another pair of 

 small wheels added at the front of the engine, 

 the formula now being oo O o. 



A bogie is really a four-wheeled truck, 

 with a pivot which allows the bogie 

 sufficient play to go round curves quite 

 easily. 



The coupling of the wheels for goods 

 traffic led to such good results that they 

 were tried for express locomotives. The 

 first were expressed as o O O, when the 

 leading bogie was used it became oo O O, 

 meanwhile the goods engine had become 

 six-coupled, thus O O O. The next step 

 in the growing up of the passenger engine 

 was the introduction of what is called the 

 4 Atlantic J: type, expressed as oo OO o, 

 and this was followed very quickly by the 

 six-coupled giant, expressed as oo OOO, 

 and later, by the " Pacific " type, which 

 reads oo O O O o. 



Whilst this method of expression is quite 

 easy on paper to indicate the wheel for- 

 mula of a locomotive, it was found rather 

 awkward in speech. An American, named 



