Shittle is a contraction of shit and shuttle. It is a track built in a very ugly way. There are several types of shittles
Also see: Railway Styleguide
Noob shittles are tracks built by noobs. There are usually only as many blocks as are needed to support the track, regardless of safety or aesthetics. In many cases, it is only a floating 1-block-wide structure made of dirt or wood with tracks on top. Noob shittles might also use other supporting nodes like sandstone bricks or bushes. Some of these shittles are incapable of any train service because they are blocked. Therefore, new users are recommended to try Advtrains in singleplayer before starting train services.
Utterly useless shittles
Some players have failed to understand even the most basic concepts of advtrains and create absurdly useless shittles, that cannot even be operated.
Underground shittles are long underground tracks built by digtrons and they weren't placed overground only because of someone's laziness.
Examples for underground shittles are the Trump Express Lines.
Single Track Shittles
Single track shittles are trains lines with only one track instead of two. These lines only have one train running at the same time (unless they are ring lines).
2-Block Slope Shittles
According to User:gabriel, trains using 2-block-long slopes instead of 3 are also shittles.
Worldedit shittles are long straight tracks made by using WorldEdit. These tracks cut through all kind of terrain violently. Most of them had been built by User:Trump. Some are also made by User:gabriel and User:orwell.