The Motor Line and its fireless soda locomotives

A Baldwin "soda motor" for the Minneapolis, Lyndale, and Minnetonka Railway.

One strange technology that gets mentioned in passing in histories of the Twin Cities streetcar network is the "soda motor".  When I first read about it, I was sure it must have just been a play on words: soda must have come from the tail end of Minnesota, right?  Further reading explained that no, it really refers to sodium hydroxide, commonly known as caustic soda or lye—not something I'd want to spend any time close to if I could avoid it.  But it really did get used here, even if the 'Sota / soda similarity seems a bit punny.

The Minneapolis, Lyndale, and Minnetonka Railway was built in part to capitalize on tourist traffic heading from Minneapolis to Lake Calhoun, Lake Harriet, and eventually Lake Minnetonka.  Their line ran from downtown Minneapolis along Marquette and Nicollet Avenues to 31st Street South, where it turned west until reaching Lake Calhoun.  From there, it used a largely private right-of-way to run to Lake Harriet and eventually out to Excelsior on Lake Minnetonka.

The railroad started out in 1879 with one conventional steam locomotive and two "steam motors" which were small locomotives wrapped in streetcar-style shells—reportedly to help prevent horses from being frightened as trains ran along city streets (the city had ordinances against running conventional locomotives on streets as well).

Because of the steam motors, the ML&M was nicknamed the "Motor Line".  They were able to sustain operation for a while, but unfortunately the company had been the fourth and last railroad to reach Lake Minnetonka and were never very financially stable.  There was also an ever-increasing push to get steam-powered vehicles off the streets of Minneapolis, whether they looked like streetcars or not.

In December 1885, the Motor Line began experimenting with electrification using a system from Charles Van Depoele, but electrified traction had not yet matured enough by that time.  The electric motors in the initial vehicles vibrated badly and there were other mechanical problems showing that electrification wasn't quite ready for prime time.

In 1886, the railroad began running soda motors manufactured by Baldwin Locomotive Works in Philadelphia, reportedly the first (and possibly the only) such engines to be built in the United States, it seems that others may have been imported previously for use along elevated rail lines in New York City.  European railroads had been experimenting with them for some time, and it seemed they might have been a way to deal with tunnels without filling those limited air spaces with noxious exhaust. Articles on the experiments of a Mr. Honigmann near Berlin, Germany were described in the British journal Engineering and in Scientific American in 1885.

An article on Honigmann's locomotives from the British journal "Engineering".

The idea of a soda motor conjures up childhood memories of baking-soda volcanoes: Mix baking soda with vinegar, gases get produced, and in seconds the mixture bubbles up and overflows its container.  However, while these locomotives mixed soda and a liquid (water), they didn't use gas directly generated by chemical reactions, but rather operated by capturing the heat that was generated.

Caustic soda is exothermic when it comes in contact with water.  Given a large enough quantity, this soda and water solution can generate enough heat to boil water and produce steam.  Instead of having a firebox filled with coal or another fuel to heat up a boiler, soda locomotives had boilers jacketed with a container filled with sodium hydroxide (or other salts in some cases).  The heated soda caused steam to be generated in the boiler, the steam was fed through pistons to provide propulsion, and then the condensing steam was fed through the soda to cause more heat to be produced.

This normally operated in a closed loop, so no steam was escaping the engine during operation (a side-effect being that the engines were practically silent).  Only when boiler temperature reached high enough levels where the soda solution itself began to boil would steam be let off.  However, operating a soda locomotive was a race against time:  Eventually, the sodium hydroxide would become saturated and wouldn't generate enough heat to produce more steam.  Nonetheless, these locomotives were able to operate for hours at a time before being brought back to home base to be recharged.

At the end of operation, a stationary boiler (likely installed at the railroad's roundhouse, the site of today's Nicollet Garage) would be hooked up to the engine and feed superheated steam through the soda solution, boiling off the water, leaving more or less solid sodium hydroxide, and allowing the process to start all over again.

Clearly this wasn't the most efficient process.  According to the 1885 reports on Honigmann's locomotives, the soda motors only produced about 60% as much steam per unit of coal as traditional locomotives did, though the stationary boilers could be run on cheaper, lower-quality coal than their mobile counterparts and (at least theoretically) didn't require as many employees to run.

Still, the prospect of an exploding soda motor spewing (literally) tons of boiling-hot caustic material around a neighborhood means that it was probably good that the technology never really caught on.  Minneapolis passed an ordinance in 1887 which banned the soda motors.  Van Depoele had apparently continued work on making electric traction more reliable on the line, but unfortunately the company had gone bankrupt.  It quickly came under the control of Thomas Lowry's Minneapolis Street Railway, and within a few years the entire streetcar network in the Twin Cities was electrified.