New Bedford - Wm Mason
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New Bedford Railroad - Wm. Mason 0-6-6T

While credited in most books as being built for the New Bedford Railroad, the article linked to later in this section indicates the William Mason was built of the Boston, Clinton, and Fitchburg RR.

Source - Keith Christenson collection.  Click photo for a larger version.  Click here for the source article from the Dec 1900  'Locomotive Engineering'.
An engraving of the William Mason, from the appendix of Graheme Hardy's 1950 reprint of the 1883 "Recent Locomotives".  This engraving contains a fairly detailed set of specifications.  Click engraving for a larger image.
Name Wm. Mason Number 23
Works No 536 Type 0-6-6T
Date 1874 Drivers 3'6"
Cylinders 16x24 Weight 73,920
Gauge 4'8 1/2"    
Later RR  

According to Abbott, this engine was built for the New Bedford Railroad in 1874.  It is a standard gauge locomotive built in 1874.  It was the first locomotive with Walschaerts valve gear in the United States.  

According to David Fletcher, "the Walschaerts had a big advantage over the Stephensons in a Bogie application. The 'cut off' of the steam to the pistons is controlled by the Johnson Bar in the cab.  It works a lot like a gear shift ... as you pick up speed, you bring back the Johnson bar a couple of notches, limit the steam going to the cylinders and go into a 'coast mode'. The movement of the Johnson bar relative to the valve gear is touchy.  Now, if the loco is articulated and the loco chassis moves relative to the cab ... BIG PROBLEM! Every time you turned left you let in more steam to the cylinders ..and speed off, every time you turn right, you cut off the steam, and you slow down ... and that is without touching the Johnson bar ... that is because the distance between the Johnson bar and valve gear changes as the bogie pivots...get the idea? Mason used some clever alignments to minimize the problems with the Johnson bar/ Stephenson valve gear alignment.

The Walschaerts valve gear is a nice simple external mounted mechanical ... and by placing the lifting gear to the valve gear, over a boiler mounted rig, connected back to the Johnson bar, the loco's chassis could pivot with no discernable change in distance between the lifting rod and Johnson bar.  PROBLEM SOLVED!!

That is why all the Walschaerts type mason Bogies had the linkages over the boiler top.  The lifting bar is right near the bogie pivot point.  The system works!"

(c) 2007 Iron Horse 1:29