Round Trip puzzles were invented by Stitch (his initials are S.E.W.) and first published in Dell Champion Variety Puzzles in the early 1990's.
Inspired by Stitch, I have extended the Round Trip idea to new grid sizes and geometries, and have published these puzzles in magazines, calendars, as a book "Round Trip Puzzles" (Sterling, 2011), and most recently as a puzzle app called Monorail for iOS and Android (app written in collaboration with my son Aaron B. Iba, and ported to Android by Apportable).
Based on my book "Round Trip Puzzles" (see below)
App programmed by Glenn Iba and Aaron Iba (IBA Puzzles)
Published by Puzzle Wright Press/Sterling Publishing January 4, 2011.
Book includes 212 new puzzles of various sizes (ranging up to Square 12x12 and Hex 11x11).
Expanded Book Introduction (online only) with in-depth solving strategies
Click the image to play these challenging path puzzles online.
One-Way Trip puzzles are a variation of Monorail Round Trip where there are Start and End Stations.
Click the image to play these challenging path puzzles online.
There are some strategy hints and more path puzzles in my paper Hamiltonian Cycle Puzzles submitted to the Exchange Book of G4G8 (the Eighth Gathering for Martin Gardner).
Scott Kim's page-a-day calendar includes 12 Round Trip puzzles or variations thereof each year.
The 2011 and 2012 Calendars include both Square and Hex One-Way Trip puzzles.
The 2013 Calendar includes Monorail Round Trip puzzles (both hex and square).
Purchase Mindbenders 2011 Calendar at Amazon
Purchase Mindbenders 2012 Calendar at Amazon
Purchase Mindbenders 2013 Calendar at Amazon
This is a German language site, but you can click on the "English Flag" at top of the page and Google does a neat job of translating to English! I believe these puzzles are taken (with permission) from my own Grand Tour pages, but Otto Janko has created a nice interface for playing the puzzles - check it out!
Under the name "Loopy" this app includes 9 Monorail-type puzzles.
[ Not to be confused with Simon Tatham's Loopy puzzles which are like Slitherlink (a different kind of fun looping path puzzle).]
This Logic Games app includes many other fun logic puzzles (63 in all)!