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In my story, all the cities and towns have had to either build some sort of dome to cover their cities or move underground due to toxic fumes choking the air.

The air is so toxic that a breath is more than likely going to kill you.

Travelling from city to city for trade is difficult and requires breathing apparatus which is expensive to use and maintain.

Other than the cost another reason travellers dislike the current method is that small mistakes such as miscalculating the air required, or getting lost can cost you your life.

It has been proposed that a tunnel system of some kind would make for safe passage.

What would be the best long term option when creating these tunnels, and what would be the safest way to build them?

The cities they trade are quite close. Within 10 miles of each other.

I am currently considering either digging a tunnel, or building one above ground.

Another point to consider is that the materials in the ground are not known. It could be sorry, rock steel, etc..

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  • $\begingroup$ What's the technology level? This sounds almost WWI with the difficulty with the breathing apparatuses? Maybe something before, like Victorian era? $\endgroup$ – Nex Terren Aug 5 '16 at 20:00
  • $\begingroup$ The time period is just before WW1 would have taken place. In my story WW1 doesn't take place due to the difficulties caused by the smog and fumes. $\endgroup$ – Terry Aug 5 '16 at 20:03
  • $\begingroup$ Why are the cities separated? Why not just replace two cities separated by ten miles with one big city? How do they grow food? I would expect plants to draw in the toxins from the atmosphere. And of course animals would die like humans. How do the cities maintain fresh air? $\endgroup$ – Brythan Aug 5 '16 at 20:57
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I'm not sure how building and maintaining miles of tunnels or tubes with breathable air is more feasible and cost efficient than just building open air rail lines for trains that carry self contained atmospheres.

I don't just mean in materials. Isn't the risk of the tunnel or tube's failure (say a broken seal) worse than an incident involving maybe one train car? If you are starting from scratch (no existing lines), you can build them much wider than we have on Earth and accommodate massive amounts of goods and people.

Edit: I don't have high enough rep to comment on OP, but how exactly is the air in the domes or underground kept breathable?

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New York City's subway system was made just after the turn of the 20th century, so tunnels of such a scope are perfectly viable, however keep in mind that such tunnels take years of effort and resources; making tunnels is a considerable investment, the likes of which depend on governments (or large corporations to undertake). This will provide natural structure against the gas, but will only be stable if drilled through bedrock. Once you leave this, plates moving and ground shifting can cause complications, but not ones you can't deal with.

I imagine sealant is semi-regularly (every couple of years?) reapplied to the walls and ceiling of these tunnels to accommodate for any forming cracks, similar to how asphalt cracks are patched.

Also likely the tunnels are partitioned off into sections with double sets of doors sealing off the spans set up so both doors are never open at one time. This way if a leak does happen, it will be contained until it can be pumped out. This is a pretty low-technology, easy-to-manage, fool-proof way of containing such a leak.

Very small leaks I'm assuming won't be an issue; if the air is 999 parts clean air to 1 part contaminated air, I imagine the citizens won't notice it. Still, you'll want some sort of detectors placed in the tunnel (I'm assuming this is possible with your smog, although I don't know the exact nature of it) that maintenance patrols would check once each every handful of days.

Only important trade routes of relatively short length would such be affordable, and would be an investment from the cities. Such tunnels would clearly encourage trade and would be beneficial, while other non-tunnel trade routes would be limited to those who have the finances to purchase the better breathing equipment (large corporations, government), or those who are willing to risk more shoddy equipment. In either case such routes won't be your main paths of trade.

Above ground tunnels would be considerably more work to build and maintain, and your chances of leaks will be far greater (since you no longer can depend on multiple feet of dirt and stone). Underground tunnels can be dug a few feet deeper for increased security, at no practical increase in their associated expense.

Probably most people using the tunnels would still carry emergency breathing equipment (like modern cars have seatbelts and airbags) just in case a leak happened.

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  • $\begingroup$ Regarding the expense and effort: Given that these cities already have massive domes overhead, it seems these tunnels will be trivial by comparison (-1). Agree that the most important factor is ensuring multiple points of failure (+1). $\endgroup$ – cobaltduck Aug 5 '16 at 20:15
  • $\begingroup$ @cobaltduck I wouldn't underestimate the expense of the tunnels. In today's dollars, using the cost of NYC's recent routes as the cost point, a trade route between Chicago and New York City would be between 300 billion and 2.16 trillion. Chicago's tax revenues appear to be in the hundreds of millions, not billions and certainly not trillions. (Edit: Removed dollar signs because they're freaking out the formatting...) $\endgroup$ – Nex Terren Aug 5 '16 at 20:26
  • $\begingroup$ @Nex Terren: you can use \\\$ to print a \$. $\endgroup$ – John Dallman Aug 5 '16 at 22:36

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