Doesn't the current to run the trains heat the wires?

I hope not. Miles of cabling putting out enough heat to melt ice might be less than entirely efficient.

Separate de-icing systems are available, but I'm not sure how widely they're used.

Shouldn't there be a small generator onboard for emergencies, to at least keep the toilets running? Perhaps a tank of natgas for heat?

Trains have very limited space, and weight is an issue.

In the UK failed electrics are hauled to safety by troubleshooter diesels. But if a long stretch fails, the diesels have to do multiple runs to rescue multiple trains. Which takes a while.

There were plans to create a hybrid diesel/electric next generation Intercity train in the UK, but weight and space requirements made it impractical solution.

I expect when nuclear fusion becomes small enough to fit into an engine car these problems will be solved. But until then catenaries, with all of their faults, remain the best choice.

To heat the overhead wires with electricity, you have to short-circuit them, which is both a waste and dangerous. (I know of someone who did it in a cold spell, though...) But serious icing on overhead wires is a rare occurrence and there are some devices against it. However, when overhead wires snap, it usually has to do with some defect. Most often it's some failure of the system to compensate thermal contraction (like a stuck roller), sometimes material failure, and sometimes something falling on or getting stuck in the wires.

Trains have batteries. They don't last forever, though, and in modern times, even a simple passenger car comes with a sophisticated software... which may be ill designed and shut down the system in the 'wrong' kind of emergency. (I could tell stories.)

"Catenary" is American English for "overhead wire"; though recently it is increasingly in use in rail literature this side of the pond, too (I prefer to use it, too).

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Yeah, but sending out a snowplow and switching on an electric switch heater can be less fuss than repairing catenary in a snowstorm. Assuming the electric heating installation programme wasn't held up by "cost-saving" and the number of snowplows wasn't reduced by the same...

_{*Lunatic*, n.
One whose delusions are out of fashion.}

IT's probably a factor too that when a train gets stopped by a frozen switch, it's still easy to evacuate passengers to a station building nearby, as opposed to a train broken down on the open line; hence the more numerious switch failures earn less news space.

Winter chaos in train traffic - Stockholm News

One train stopped for several hours in Östegötland yesterday while the 700-800 passengers where without food or electricity. The toilet tanks onboard are also reported to have been full. The stop was due to an electric wire that had been torn down. The passengers were later evacuated to another train which later was affected by a failure in a security system and the passengers had to evacuate once again. Shortly after, another train hit an ice bloc (!) on the rail and broke. At other places trains stopped due to frozen switches.

_{*Lunatic*, n.
One whose delusions are out of fashion.}

For the last 15 years Swedish infrastrucuture has been under attack from neoliberalism, and the accumulated lack of maintenance is becoming clearer for every passing year: last winter the rail system was in shambles, and this year it's close to collapse

Allow me to snigger: you have no idea what's close to collapse :-) Here, cost-saving on maintenance is much more 'advanced', and we get the cold weather catenary ruptures (not one, but a couple in a row) already in autumn. So you still have a lot of neolib destruction ahead of you :-)

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Ah, just wait until you get to USA (#1) style maintenance and operating practices! Even though things have improved markedly from the dark days of the 1970's-80's when even cars parked in yards would derail, we still have a long, long way to go to approach European levels of service quality.

One of my favorite railways; the Malmbanan was so comfortable with catenary and snow that they used one of the very few overhead powered rotary snowplows.

The South entrance to Stockholm is a frequent failing point. The basic problem here is that Stockholm rail was originally planned for about ten trains a day. So instead of running the rails north and south around Stockholm and leading the rail to Stockholm into a cul-de-sac Central, all rail runs through Stockholm making in expensive and politically trying to expand capacity enough. And local rail runs on the same tracks. This is not only shit happens, it is a 19th century built in weakness that it takes to much political will to overcome.

Add under-budgeted maintenance and neoliberal ideas about planning and even in the best of years you get delays in winter. And this winter looks like another though one.

Sweden's finest (and perhaps only) collaborative, leftist e-newspaper Synapze.se

For DB, we had the same in summer, too, with air conditioning not scaled for 40°C outside temperatures... (though that cost-saving idiocy was still not as bad as for the refurbished commuter train coaches around Budapest, which have air conditioning with a power insufficient at any positive temperature difference, as the heat input from direct sunlight was apparently not taken into account...)

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Thanks for the great links!

Heat the catenary.
It could be necessary to heat the overhead wire at some locations during extra tough weather conditions. Sometimes it is sufficient with the heat generated by the traction current why a frequent traffic is preferable from this aspect. Sometimes an additional reactive current is used to prevent or reduce the build up of rime on the contact wire.

So the Nordic countries indeed do it, too, albeit no details are given.

_{*Lunatic*, n.
One whose delusions are out of fashion.}