Transport, Rail, Unintended Consequences, and Cascade Failures
In researching various transport efficiencies over the past few days, the question of when "cab forward" locomotive design came into being occurred to me. The first steam locomotives placed the engineer's cab behind the boiler largely to facilitate access to the fuel tender which ran behind it, though this compromised other aspects, including both forward visibility and greater exposure to fumes. That was a particular problem in railroad tunnels: being in an enclosed space with a large, hot, burning thing, and other elements, can be less than pleasant....
Modern diesel and electric locomotives virtually all place the engineer's cab at the front of the locomotive. This improves foreward visibility, but can compromise safety (there's less mass between the crew and any potential front impacts), and communications between forward crew and those further back on the train (a greater issue with cab-forward steam engines) can be somewhat worse.
The Wikipedia article on the topic describes consequences encountered when switching cab position in combination with other effects. I'm familiar with multi-stage cascade failures in aircraft and industrial accidents, this is an interesting example of same for railroads:
Accident near Hasson, California, November 19, 1941:
All of the cab-forwards were oil-burning locomotives, which meant there was little trouble involved putting the tender at what would normally be the front of the locomotive. The oil and water tanks were pressurized so that both would flow normally even on uphill grades. Visibility from the cab was superior, such that one crewman could survey both sides of the track without difficulty. There were concerns about what would happen to the crew in the event of a collision, and at least one fatal accident occurred on the Modoc Line in northeastern California when a moving locomotive struck a flat car. Turning the normal locomotive arrangement around also placed the crew well ahead of the exhaust fumes, insulating them from that hazard. One problematic aspect of the design, however, was the routing of the oil lines; because the firebox was located ahead of the driving wheels (instead of behind them, the usual practice), oil leaks could cause the wheels to slip. A nuisance under most conditions, it resulted in at least one fatal accident. This occurred in 1941 when a cab-forward with leaking steam and oil lines entered the tunnel at Santa Susana Pass, near Los Angeles. The tunnel was on a grade, and as the slow-moving train ascended the tunnel, oil on the rails caused the wheels to slip and spin. The train slipped backwards and a coupler knuckle broke, separating the air line, causing an emergency brake application and stalling the train in a tunnel that was rapidly filling with exhaust fumes and steam. The oil dripping on the rails and ties then ignited beneath the cab, killing the crew.
Cab-forward design puts...
Firebox ahead of driving wheels meaining that...
Oil leaks greased drive wheels leading to slip...
Inside a 7,366 foot unventilated tunnel...
On an up-hill 1% grade...
Resulting in backwards-slippage...
With the additional factor of a broken coupler resulting in...
A separated (brake) air line. At which point it should be noted that the air brake itself was a significant and crucial safety innovation patented by George Westinghouse in 1868, notable in that positive pressure was required to keep the brake released, and that the system would fail with brakes engaged on carriages behind the locomotive, preventing the train from safely rolling backward out of the tunnel. Which means that another factor I've discussed previously is involved...
Safety and/or redundancy systems can themselves contribute to disasters, as...
Application of brakes results in train and engine stopping in the tunnel unable to either proceed (wheel slippage) or reverse (blocked by braked carriages)...
Said tunnel is rapidly filling with both smoke (consuming oxygen and constributing both CO₂ and carbon monoxide, as well as particulates), and steam (hot, hot, hot), while....
The dripping oil lands on wooden (and creosote-soaked) ties, ultimately igniting.
Result: Loss of locomotive, rolling stock, cargo, several crew, and two others. Five killed, four injured.
(Two of the dead are described as "trespassers".)
Full Interstate Commerce Commission accident report (PDF)
It's interesting to consider at what stage the accident was inevitable. Insufficient tunnel ventilation, use of wooden ties, and lack of a viable evacuation capability all likely doomed the crew. The crew if not the engine would likely have been saved had the incident not occurred within a tunnel. The slippage wouldn't have happened had the tunnel not been on grade. Routing oil lines differently would have placed leaks over ties only, not wheels and rails. Maintenance and/or engineering might have prevented the coupling failure. It's worth noting, by the way, that rolling stock is frequently not owned by railroads but instead by shippers and/or car-leasing companies, meaning that railroads are at the mercy of those parties' maintenance procedures.
From the full accident report, the train had already slipped and stalled 5,000 feet prior to entering the tunnel.
The Interstate Commerce Commission's report describes the effects of the fire:
...the fire resulted in considerable damage. The cab was badly distorted and all appurtenances in the cab were badly damaged. The No. 2 axle of the engine truck was bent and the engine-truck spring rigging was badly damaged. The wheel centers of the No. 1 pair of driving wheels were loose and the brasses, shoes, and wedges were melted. The brasses and the liners in the journals of the No. 2 pair of driving wheels were melted....
The train's fireman had little experience, being hired August 29, 1941, less than three months prior to the accident.
The front brakeman (among the survivors) had noted the left cylinder cock of the No. 1 engine "blowing" continuously between the trip's origin at Los Angeles Yard and the tunnel, but his suggestions for repair were overruled by the engineer (among the casualties).
The report's conclusion: "No defect was found which could have contributed to the cause of the accident."
I'd argue that the Commisson's conclusion doesn't match its own findings of fact.
More on air brakes: