This past weekend, my local LEGO train club (TVLTC) attended Railfest, a special railroading event at the Tennessee Valley Railroad Museum in Chattanooga, Tennessee. To say the least, it was an interesting weekend. Our first attempt at an outdoor LEGO train exhibit was made somewhat more challenging because the remnants of a tropical storm happened to be sweeping through town that weekend. But I digress...
The Tennessee Valley Railroad Museum depot is situated in the center of a large wye. Over the course of the weekend, numerous excursion trains used the wye to reverse direction. That got one of TVLTC's club members, Trey, thinking about building a wye on the next TVLTC layout.
In principle, a wye sounds like a great way to intersect multiple rail lines while permitting reversing. However, the LEGO version is challenging to execute. First, recall that any track configuration that permits a train to reverse direction (such as a wye or reversing loop) will create a short circuit. This isn't such an issue for Power Functions trains, but it does impact 9V engines.
Perhaps more importantly, a wye takes up a lot of space! Here's a sketch of a LEGO wye that I did using BlueBrick. For size comparison, I also sketched a reversing loop. You could easily fit the reversing loop inside of the wye! In real life, a wye would typically take up less space than a turning loop. However, given the relatively narrow curve radius of LEGO train track, a reversing loop can be executed more efficiently.
The other interesting aspect is that for purposes of reversing a train, the length of train that can be reversed on a wye is limited by the length of the spur tracks coming off of the wye. This was evident while watching trains reverse on TVRM's wye. Trains were limited to 3-4 cars long when reversing on the wye. A reversing loop is more efficient; the length of the entire loop, unrolled, dictates the length of train that can be reversed.
I'm not trying to deter anyone from building a wye. If you have the space, go for it. However, a wye is quite difficult to implement elegantly given the limited track geometry available to LEGO train fans.