• Segments: Add a tiny torch alcove every 6 blocks (meters). Alternate left and right sides.
• Six blocks is easy to count when digging.
• Torches every 6 provides ample light (more than minimum to keep away mobs, but it keeps everything bright and less claustrophobic)
• I call each block of 6 a "segment."
• It's easy to count segments later when walking the tunnel to compute longer distances.
• Suggested depth: Around level 28-31
• This is because gold is found on level 32 and below, so you can find bonus gold while you're tunneling!
• This depth also means that almost every cave you find is going to be a good, deep one.
• I started out a subway at level 31 (that is, mining out levels 31 & 32 - see levels page for terminology), but when I tunneled under the ocean I was surprised when the ocean foor went as deep as 34-35. So I had to drop three levels to 28. (Gamepedia claims that the usual depth of an ocean biome is 48. Obviously they can go WAY deeper. And with gravel on the bottom, you can easily dig underneath the gravel, letting it fall into your tunnel and flood yourself out! So I'd say use 28 if you're planning on tunneling under an ocean. And YES, you CAN find hidden caves under an ocean!)

• Station size: 11 x 3 x 5 (W x D x H)
• The station has to be large enough for you to see it, and have time to jump of a minecart, if you are flying by at top speed.
• The station also has to be large enough to accommodate a large letter or number identifying the station.
• The station should also be a comfortable underground room instead of a claustrophobic tiny chamber. Think of the subway as your critical transportation artery, not just a dark hole in the ground. You are Steve, with awesome creative powers. Make it spectacular!
• Large identifier: 3 x 5 (W x H)
  • This size accomodates digits 0-9, as well as most letters, using two different colors of blocks.
  • Stained clay makes the best identifier, but you can use anything that will stand out. Even jack o' lanterns!
  • See screenshot for example.
• Station spacing: About every 25 segments, +/- 5 segments
• So far, I have consistently found new caves at this interval. (Once I went 50 segments without seeing a cave, so I went back 25 segments, built a station there, started digging around, and eventually found a nearby cave. That small cave connection we almost missed ended up connecting to one of the largest caves we've ever found, with 20+ diamonds!)
• If you find a cave in a different place than you want the station, say 21 segments along instead of 25, put the station where you want it, then make a foot path tunnel (non-railed) that parallels the rail tunnel back to the cave. Then your station will provide access to the cave. Optionally, put walls around the rail tunnel to block access to the cave from there (making sure there's a way over or under that tunnel in case the cave is on both sides). Use cobblestone so you know it's an intended wall that you don't want to re-mine later.
• If the cave is more or less where you wanted the station anyway, put it there to reduce the number of blocks you have to take out (you may be doing some "filling" in that case).
• If desired, add a staircase from the subway station back to the surface. (Each station should lead to a cave and/or the surface.) Be careful about hitting a lake or river on the way up (see the "planning & mapping" section).

• Surface stations: Place towers
• Because the subway is totally invisible to the landscape, you will want very visible towers above every staircase that leads down to a subway station. This way you can find your way back to the subway, and then home, even at night.
• Survival mode hint: Jump up, and, while still in the air, place a dirt block directly below you. Then place four cobblestone blocks around you to start making four columns. Repeat this to start making a tall tower. When the tower is done, use the dirt stack to get down to the ground safely by removing blocks from below you one by one.

To create the parallel Creative Mode world, click "Play" then "Edit". This is normally the screen where you delete a world, but you will also see all the numerical seed values shown for each world. Write down the number for your world. Then cancel, create a new world, select Creative Mode, click "Advanced", and punch in your seed.

To measure distances, fly up in the air above your starting point, point yourself toward your destination, then fly forward and start a stopwatch at the same time. When you get to your destination, take the time in seconds and multiply by 1.85. This should be a fairly accurate estimate of the number of segments you need to make underground to get to your destination. (Multiply again by 6 if you prefer the distance in meters.)

You can also use this distance measuring method to map out your entire world. In the example map below, each square represents 20 segments. What started out as a relatively "boring" world is now very interesting due to our using the parallel Creative Mode world to find the mesa biome. Because it is so far away, over a period of a few weeks surface rail transportation was added so there's now a way to collect resources from the mesa and bring them home efficiently. The new subway being constructed provides access to more of the mesa as well as some very rich caves. Eventually it will be extended all the way back to the spawn point; the map is a good way to plan this route.



(By the way, the map above depicts the same world that the 3-D Pac-Man House and 13 minute surface rail ride loop are in.)

Miscellaneous screenshots: When the tunnel hits a large cave, or the ocean floor:

 


 

Happy tunneling!