#104: Rail Pool — "The Longest Train Journey"

I’ve set myself a little puzzle construction challenge. How tall can I make a really narrow Rail Pool grid before running out of ideas? Apparently, the answer is 200 rows.

Okay, I didn’t quite run out of ideas by the time I wrapped up this puzzle, but I was spending increasing amounts of time on elaborate ideas that just didn’t pan out with my self-imposed constraints which was getting a little frustrating. Those constraints were: only 5 columns, no number clues greater than 4, no region with more than four clues, and (mostly) only rectangular regions. I think the only exceptions to that last point are larger regions with a rectangular bounding box that contain some smaller rectangular regions inside of them. Breaking that last rule in particular would open up many more possibilities.

The puzzle is generally meant to be solved from top to bottom, though you’ll occasionally have to look a handful of rows ahead and then connect back up. I didn’t check if there are any additional break-ins further down. The difficulty probably vaguely increases, but it’ll also bounce around a bit every now and then. There are a few sections that assume some familiarity with loop theory.

When you get to question mark clues, you’ll be halfway there. In particular, the puzzle consists of 100 rows using only number clues, 50 rows using only question mark clues, and then another 50 rows using a mix.

Rules: Draw a non-intersecting loop through the centres of all cells. Some boldly outlined regions contain number clues. If a straight loop segment visits any cells of a clued region, its length must match one of these numbers. Each number must correspond to at least one such loop segment. Question marks represent any positive integer, but numbers cannot repeat within a region.