It's not the end of civilization, but it's still really bad...
...because the modern world is a massive interconnected web that's already beginning to have a cascade failure and the snake just knocked over more dominoes.
Losing one major city is a humanitarian crisis that can be recovered from. New Orleans wasn't flattened when Hurricane Katrina hit, but was effectively turned from a productive city into a massive disaster zone.
What we're looking at here is worldwide destruction in cities that was only exceeded by the massive bombing campaigns (plus 2 nukes) in World War II. Although the total devastation is less, the selection of targets makes things far worse even in areas that escaped the steamrolling.
Multiple important capitals were vaporized. The nations most likely to be able to coordinate a global rebuilding campaign lost most of their leadership as well as the central offices of all those departments of various governable things.
Every oil refinery and every major metal smelter in every country is gone.
The good news is that most electrical plants that weren't blown up, steamrolled, or otherwise taken out as collateral damage would be coal or natural gas fired.
The bad news is that mining and moving coal uses a lot of gasoline and diesel burning equipment. Petroleum is being rationed, but once the last tankers, freight cars, and trucks full of refined oil products are unloaded, it will take many years before supplies get anywhere near 2017 levels.
The US and areas of Eurasia bypassed by our rolly polly snake o'doom probably breathed a sigh of relief, but that was premature. That 400 meter wide swath of destruction took out plenty of high voltage power lines as well as pipelines for oil and natural gas. Power grids must balance load and demand or else very big circuit breakers pop and very big fuses blow. Much of the US as well as significant areas in other countries would be blacked out.
The integration of the US grid is gone. Places near hydroelectric dams and nuclear plants should be back online fastest. Wind and solar were avalable then, but on a much smaller scale. Natural gas powerplants could be reactivated, but with so many damaged pipelines, many will have no way to resupply.
With little or no power, more and more municipal water plants will go offline. With no fuel for cars and trucks, repairing and resupplying all forms of utilities will rapidly become... extremely difficult.
Unfortunately for the people of Earth, there are very few who don't in some way rely on oil and electricity. A lucky few places might have a natural gas field with an intact pipeline connecting to a power plant. Others may still have power from nuclear or dams. The problem will be that even the megadams or biggest nuke plants can only produce so much power. Those left in the dark will migrate to the places with power, and eventually even strict rationing won't be enough to prevent rolling blackouts.
But wait, there's more!
Yes, if you are a good farmer with modern equipment, you can feed 100+ people with your labor. How will your crops reach those people? What happens to your yield without the top of the line hybrid seeds, without the high end fertilizers and pesticides, without the gasoline powered plowing, planting, and harvesting equipment?
The poorest farmers in the poorest countries will be the least affected, but even they will miss the benefits of the electric and gasoline powered civilization, even if the nearest power outlet is a 3 day walk away.
Assuming no mass looting and no epic battles to control the few remaining reliable sources of power (because we all know how well humans come together and sacrifice their own personal desires to unite for the common good in times of crisis), we're still looking at mass starvation around the world for at least a year or two.
Once a few smelting plants are online, coal fired steam locomotives can begin to move critical goods (including more coal). Natural gas lines can be repaired, getting a few more power plants back online. Others can by converted to burn coal (probably less efficiently than plants designed for coal in the first place). Oil refineries could eventually be rebuilt.
Rebuilding will be a long, slow, painful process. Civilization won't completely collapse, but getting back to 1950's level of living (with weird little bits of newer tech scattered about) in the more advanced countries woul take at least a decade, probably more.
My very rough guess is that we're looking at losing around 70-80% of global population between 2017 and 2022 due to direct and indirect effects.
Whether you think my estimate is too optimistic, too pessimistic, or just right, there is one more factor to consider.
Was steamrollering selected cities the final act in Jormungandr's performance, or would the snakebot go after some new target? If all the nuke plants and all the major hydroelectric dams went, that woul make rebuilding far harder. There are other targets that wouldn't be as immediately devastating, but that would still cause massive problems.
We have reason to believe Jormungandr has some sort of visual abilities and can likely read our languages. Large signs (and local radio transmissions) near wherever the snakebot is parked offering to discuss things might not be the worst idea to try. It would be best if whatever was left of the local government there seized all the radios. One hundred voices yelling different things won't help. One voice saying "Let's talk. What do you want?" might prove to be the key to preventing another wave of destruction (and, if not, maybe only the lone guy standing out there all by himself gets zapped instead of a globe-spanning reaction).
Edit: Let's play with casualty numbers a bit.
Excluding those killed directly, there would be many collateral deaths during the first month. The initial nukes were mostly clean (mostly), but even in the event of zero fallout, there will be a lot of people with flash burns (anywhere from severe sunburn all the way up to being partly cooked on one side). There will be numerous injuries from flying glass, lots of injured survivors in damaged and collapsed buildings. Don't forget the drivers (and pilots!) who were close enough to be temporarily or permanently blinded. Then there will be people in New York who will die from lack of medical care, plus those injured or killed when the railguns take out refineries and metal smelters. The continent crossing rolls will break power grids, taking a bad situation with electricity into a dire situation. So many injuries and so few hospital beds, and even fewer with reliable (or any) electricity. If it's an average winter in the northern hemisphere, there may be enough time to get people together into shelters to limite the number of deaths by hypothermia.
Although horrific in numbers, in terms of percent of total population, some luck and organisation could keep total deaths (attack, direct collateral, and consequential) within the first 30 days to "only" 1% of global population. That doesn't sound so bad until you realize that's about 75 million people, and is a best case scenario.
But it's still winter. Things begin getting really ugly in February.
Deaths in 1 year? (copied from my comment with a bit of editing) I'd think some areas of the world would already be having massive problems due to the fuel shortage even before Jormungandr decided to crush cities and roll through all those pipelines, rail tracks, powerlines, etc. Let's be VERY generous about governments (even in nations now missing their Capitols) doing all they could to conserve petroleum products and to stockpile and ration food. Even with that, harvests will be far down and things just keep getting worse. By January 2018, at least half of humanity is dead, and winter in the Northern hemisphere is far from over.
As above, I'm sticking with 5 years being a total reduction of 70-80% of total population? Why? In my mind, those numbers are an absolute best case scenario. Think about it. If MontyWild decides 99% of humanity is dead in the first year or two, it really doesn't matter what the doom snake does next. It will either wear itself out killing off smaller and smaller pockets of survivors or it will just hang out to see if humans can find a way back from the edge of extinction. Either way, that's not as exciting as leaving more than enough humans to rebuild civilization in a decade (or 2 or 3) with plucky young heros digging through printouts of Wikipedia pages (there won't be a lot of internet left after a year) to try to figure out what it will take to prevent the snakebot from delivering yet another unwelcome message to those who survived so far. Or, perhaps figuring out its next target in time to leave a very surprising welcome gift, like a large set of thermonuclear landmines.
