You can buy rail laying jigs, but through experimentation I have come up with this idea. I cut several small blocks that hold the rail down and in gauge while the rail is being glued. The hard part about code 40 rail is that the rail tends to fall over side ways when you let go of it.

I made these from a piece of left-over Oak wood. I cut them to about 1.5 inches in length and about 0.5 inches in width. For curved rail I use ones that are even skinnier (around 0.25 inches). Height is immaterial. Next I use a razor saw to cut the grooves that perfectly match the rail head of Micro Engineering's code 40 rail. I use an NMRA gage to make sure the grooves are correctly spaced.

If the grooves didn't come out right, I simply cut new ones on the other side of the block and marked the original side "BAD".

As can be seen on the block near the top of the photo, I made two pencil marks indicating the approximately location of the edges of standard ties. This helps keep the rails centered on the ties.

It is a good idea to make sure that the grooves are cut parallel to the track. This helps prevent crooked rails down the road.

A word about wood. I prefer to work with wood, however, wood expands and contracts with changes in temperature and humidity. I therefore keep these blocks in a room in the house that has a very stable temperature. It doesn't hurt to check the gauge from time to time.

In the photo on the right I am doing a "dry-fit", marking a piece of rail to be cut to length and also where the rail's feeder will be placed. Placing weights on the blocks really helps to keep the whole assembly steady.

The nice thing about this system is that, when you discover you need more, you can just make more. The more you use, the better the track alignment will be, especially in curves.