Some potential for "hot rocks" in railroading

A set of switches and crossover tracks along BNSF's double-track mainline west of Minot, near Lonetree, North Dakota (Google Maps)

While looking around on Google Maps to better understand some aspects of railroad operation, these little white blobs began catching my attention in aerial shots. After a while, I saw through Street View that they are propane tanks. Okay, maybe fuel for backup generators? Seems strange, but okay.

Then, the other day, I went back to look at some of the infrastructure BNSF Railway built in North Dakota about a decade ago. The whole 100-mile stretch between Minot and Williston received major upgrades and became fully double-tracked. At the site above, they installed fairly high-speed switches featuring movable-point frogs, which allow faster train operation with smoother ride quality while reducing the chance of derailment.

There are four switches here (or turnouts, as they're sometimes called), and 8 spots where there are "switch machines," the motorized units that push or pull on a piece of rail, and also include interlocking components and sensors to ensure the switch actually moved into the correct position. Switches usually have one of these motorized units, just for the main switch points, but the movable frog also has a motor attached in this situation.

Moving a set of switch points back and forth does take some effort, but why in the world would you need four 1,000-gallon propane tanks for that? Looking them up, I saw they mostly used standard 110-volt or even 24-volt power, for which that amount of stored energy is extreme overkill. After a while, I finally remembered it's all about snow and ice in the winter, and the heat needed to fend that off.

Metra in Chicago famously uses open-flame heaters on certain switches in their network, particularly those at the busy A-2 interlocking near Metra's Western Avenue station. There, a set of three tracks crosses another set of four tracks at a shallow angle, each intersecting with double-slip switches.

The rear end of a commuter train that has just passed through the orange flames shooting up around switches at the A-2 interlocking in Chicago, with the control tower visible to the right (Metra)

There are multiple different ways of heating up rails and other components that make up switches, including direct flame like this, resistive nichrome wire attached to the side of the rail, and forced air systems that use blowers to push air across gas, electric, or infrared heaters and duct it throughout different parts of the switch.

It seems that forced-air systems are the most common, and that's the type of heater at the installation in North Dakota. Many forced-air heaters have thin gray ducts that run lengthwise through the switch area, and the blower/heater assembly and its main output duct sit off to the side of the switch.

Ducts running lengthwise between the points of a railroad switch (Rails Company via railsystem.net)

Diagram of a switch heater system. 1) Heater and blower. 2) Main duct 3) Distribution duct sitting under the tracks like a railroad tie with small angled ducts heat the tips of the switch points and 4) parallel ducts running lengthwise to heat the track bed and rails. 5) Switch motor being heated by a duct outside the rails (Rails Company via railsystem.net)

Metra says that the A-2 interlocking area is too tightly packed for forced-air heating systems to work. I'm skeptical, since the track is on an elevated embankment, and something could probably be piped underground, but it's certainly true that the standard setup with heater/blower units sitting off to the side of the tracks would be a no-go at that site.

Someday, those double-slip switches and their fiery heaters will disappear when the A-2 interlocking finally gets replaced with a flyover (a project that should also add another West Loop Metra station). Switches in other locations will still need to be heated, however.

I count 22 of the 1,000-gallon tanks at the BNSF crossover site, which represent a pretty massive amount of energy. Other similar sites seem to sometimes have 24 or even 32 tanks—three or four for every switch and frog. These types of tanks get filled to about 800 gallons, leaving room for fuel to expand and for vaporization to occur. Propane tanks are recommended to be refilled once they get down to 20% full, otherwise they're susceptible to freezing up, so that gives about 600 gallons of usable energy per tank.

A 1,000-gallon tank is big enough to supply a house with heat and hot water for a year or more, so it's stunning that a single heater may require four times as much fuel. While a large house might have a 100,000-BTU/hr furnace (about 29 kW), switch heaters may be rated at 400,000 to even 900,000 BTUs/hr (117 to 264 kW). At full tilt, one could be consuming up to about 10 gallons of propane per hour (propane contains roughly 91,500 BTUs of energy per gallon).

Steel rails are enormous heat sinks, of course, with these rails likely weighing 132 lbs per yard or more on their own, and being able to rapidly transmit any heat that gets dumped into them. While a lot of heat is needed to counteract that, these sites may also be sized according to some worst-case temperature conditions that affect the fuel itself.

Propane has a boiling point of −44°F (−42°C), so it's typically a gas at room temperature. Since propane tanks are pressure vessels that contain the boiled-off gas, the container is able to reach equilibrium at several to a dozen or so times atmospheric pressure, with most of the fuel staying in liquid form.

However, as the outdoor temperature drops, the liquid boils less readily and less vapor is able to be fed into the burner. As the temperature declines from 30°F (−1°C) zero to −20°F (-29°C), the vaporization rate drops by 90%, so only about 1 gallon worth of fuel becomes available as a gas per hour. Of course, vaporization devices exist to preheat the fuel, so it's more likely that these mini tank farms are sized for the seasonal heating needs.

Are there ways we could avoid burning stuff to keep these pieces of track free of ice? It's a lot of heat, although perhaps not quite so crazy considering the size of the area. The whole rail crossover site is more than a quarter-mile long, and covers around 4 acres, around the size of a small city block.

If only low-grade heat is necessary, heat pumps could probably work, though they would need to be tied to underground loops in order to be effective on really cold days. On warmer days, the heat pumps could pull heat from outside and charge up the underground reservoir somewhat, which would be an interesting idea to think through.

I originally started this article thinking of the growing number of companies doing above-ground thermal storage, also known as heat batteries. One that gets a lot of press is Rondo, who say their primary model of heat battery can store 100 MWh of energy, and is able to provide heat output at a rate of 7 MW, which would be ample for a site like this.

If it could be hooked up in the right way, that type of battery could provide heat for this site for several days even in very cold temperatures. Just using hot sand or rocks is one of the cheapest ways we've found to store energy. 

Propane is annoyingly cheap when comparing against green alternatives, especially in places like North Dakota, but it would be interesting to see if someone can make the financial cost worthwhile without even needing to think about the environmental benefits.

It's clear that there are technical options for making greener energy sources work for this purpose, though finding the right combination of energy transmission and storage solutions to make it cheap and robust will take some work to figure out.