How an abt System Steam Loco Works
The abt railway between Queenstown and Strahan Tasmania which had been built to carry
mineral production to the port at Strahan, was closed in the early 1960s and the track,
which passed through some of Tasmania's most scenic wilderness was dismantled.
The rail line necessarily had some very steep grades;
too steep for conventional driving wheel arrangements to maintain traction
while pulling required heavy loads, especially in wet conditions.
Five special steam locomotives
were built with a double cog drive, by which the steam powered cogs engaged with
installed racks on the steeper sections.
The locomotives are unique, and the workings
are generally a little mysterious as they are not externally readily apparent.
This web page is mainly a study and attempted description of the workings
of these locomotives, related to the way the pinion mechanism which engages
the rack on the steeper sections of track.
Click here for first picture (Pictures can be clicked on
to advance to next)
Abt loco number two at the Tasmania Transport Museum at Hobart.
Ths loco was built in 1898 by Dubs and Co., and was donated by the
Mt Lyall Mining and Railway Company to the museum in 1965.These pictures were taken
of the mechanism beneath this loco in July 2009. It had at that
time been out of service for about 49 years.
Workings of a Tasmanian Wilderness Railway
abt Steam Loco|
The rack steam locomotives on the Tasmanian Wilderness Railway, each have two cog
They are mounted between the main driving wheel axles.
They cogs themselves are not directly attached to the driving axles, but are enclosed within a
heavy cage frame, or bar frame.
Each corner of the frame rests on a big bearing.
Thus, there are two of these bearings on each driving axle
which all take the weight of the frame.
The traction being developed from the cogs being powered,
is transferred through the driving axles
via these four large frame support bearings.
Thus the axles are in effect, pushed along by the frame
which is driven by the cog wheels.
The cog wheels, connect with the rack on the steep sections as the rack is
higher than the rail height.
The cogs are are always at the same height and engage with the rack automatically.
When the locomotive is on ordinary track (not a rack section), the cogs are stationary
as are the cylinders which drive the cogs.
The cogs are driven by two extra steam cylinders beneath the smokebox in the center of the engine
(visible on front views of the locomotives)
These cylinders and enclosed pistons, act through connecting rods onto the rear cog crank.
Another connecting rod, connects the rear cog crank to the front cog shaft,
similarly to conventional steam locomotive drive setups.
The connecting rods which operate the cog wheels, are on the outside of the heavy bar frame which
sits on the two driving axles on the four big bearings as described earlier.
The middle cylinders which work the connecting rods and cogs, are on an angle and the main
connecting rods are worked through a yoke arrangement which enables the main connecting rod to
be right above the side connecting rod, obviously in order to compact the design and fit
all equipment into available space.
Because the inside cylinders are higher than the front main axle, hence the angularity to the rear
cog wheel driving crank, the slide gear and the connecting rod clear are consequently
above and clear of the front main axle.
As cogs are of smaller than the main driving wheels, a mechanical advantage or leverage, or
a lower gear is attained and gives greater pulling power where it is needed.
'If power is cut off' to the outside main cylinders and therefore to to the driving wheels
while on the rack section,
then the main driving wheels would not be driving, but would merely be carriers of the engine's
weight because the cogs which are pushing on and through.
Apparently though, according to published information
the outside cylinders also drive the wheels, while the locomotive is on the rack sections.
Because of the cog setup between the driving wheels, and the space needed for it,
the main loco frame and wheel bearings are on the outside of the driving wheels
The pictures below were taken of the preserved abt engine
no.2 at the Tasmanian Transport Museum Hobart which is pictured above.
The pictures were taken in the dark and consequently they were guesswork.
However some sense was able to be made of them and the result is this page
A view taken looking under the front of the loco above. The view is of the front axle.
The right side big bearing on the axle, one of four as there are two on each driving axle,
that carries the caging which supports the cog drive gear, can be plainly seen
while part of the left side one is visible. Visible also, is part of one of the two
cog wheels which are supported between the two driving axles
A view from below the front axle showing the two cog wheels
with the front one in the foreground. Connecting rod for the right side
attached to the crank which turns the cog wheel, can be seen to the left
of the picture, while the right side rod is out of sight and would be on
the right side of the picture.
The side rods are pushed and pulled from the crank of the rear cog wheel.
The main connecting rods from the steam cylinders, are attached to
a tongue via a yoke. The tongue, is part of the connecting rod between
the two cog wheel cranks. This setup, allows the main connecting rod to be
above the side connecting rod to save space, and this together with the
height of the center two cylinders which accounts for their angularity, allows
the main connecting rod and slide gear, to operate clear of
the main front driving axle.
Looking in under the boiler on the right side of the locomotive. To the right,
can again be seen the front driving axle and, this time, the top of the
left side front big bearing which supports the cog operating gear,
both of which were seen in picture 2 above, Oil reservoirs are on top of
these bearings, and on top of the cog wheel bearings, each side of the cog wheel,
can be seen other oil reservoirs although under a film of dust and grime.
The top of the front cog wheel is plainly visible, although covered with a
generous coating of dirt and grime.
Also visible is the slide gear for the left side
main connecting rod, with the rectangular oil reservoir on top. Beneath this,
is the front of the main connecting rod which connects to the
top of the side rod at the rear cog wheel crank.
The front end of this side rod which operates the left side of the front cog
wheel via the rear cog wheel crank, is visible.
It appears as if provision for the lubrication of the ends of the connecting
rods, is via a grease gun.
A view from the same place as previous picture, but
showing the push/pull rod from the
internal cylinder on the left side of the locomotive, as well as the
slide gear on the left and the top of the slide gear on the right side.
Looking towards the rear of the locomotive, both cogwheels can be seen
as also can be seen the left side connecting rod, the one with elongated
holes in it,
and the main connecting rod which is above it. Around each side of each cogwheel
can be seen what appear to be contractable bands around the outside sections
of the cogwheels and are likely to assist in the braking of the loco and train
on the steep down grade sections.
A view taken above the rear cogwheel. The unique way the main connecting rod
is attached to the side rod, as explained in picture three above, via a yoke
and a tongue from the side rod is shown above. Also shown in this picture, is
the back end of the right side, side rod.
A view of abt loco no 1 which has been restored to operation at Queenstown
Tasmania, showing the outside frames and support locomotive wheel bearings.
Example of a section of rack railway on display at Queenstown Tasmania
Front view of abt loco number 1 at Queenstown, showing the central
position of the
internal cylinders beneath the smoke box, that operate the pinion
Restored ex Mt Lyall Mining and Railway Company steam locomotive,
no 1 of the also restored West Coast rack
and pinion Wilderness Railway Tasmania Australia, outside the station building