Tie plates are a visible detail on S-scale models, I feel. In the real world, they are used to tie the rail down to the ties, and more importantly, to keep the two rails in gauge. When you watch close-up videos of a train passing over a section of track, you will see the rails move up and down under the weight of the equipment. That is perfectly fine. The tie plates, and the spikes that are used to hold the tie plates in position, are primarily there to keep the rails in gauge. While extremely time-consuming, I have decided to scratchbuild my own tie plates (I started doing this in late-2014). To make these tie plates in quantity, I use one scale 1"x12" Evergreen styrene strip, two 1"x2" strips, some weights, styrene glue, and a section of the rail that I will be using on the layout. The wider strip of styrene needs to be wider than the base of the rail being used, plus the width of the two strips of the other styrene, so that may vary depending on the code of the rail you use. The piece of rail needs to be longer than the 1"x12" styrene strip.
To start the actual construction, I put the 1"x12" strip on a smooth, flat surface (a sheet of glass in my case), supporting the entire strip. Then, I place the rail on top of the styrene strip. Using the metal weights, I make sure that the rail is in the center of the styrene strip all of the way. The rail may bend a little, but the weights will help to straighten it out. Use as many weights as are necessary.
I then place one of the 1"x2" styrene strips up on top of the base styrene strip, and carefully push it against the base of the rail. If you look at a prototype tie plate, you will notice that it has a lip that is positioned against the rail base. That is what this strip of styrene is trying to simulate. The styrene strip may not entirely fall against the base of the rail, but that's OK. Note that if the strip isn't perfectly against the rail along its entire length, applying glue to the center to start the process of attaching it, could cause the glue, due to capillary action, to run all the way under the entire strip. This led me to some wasted styrene.
Thinking about how to solve this, I came up with the idea shown in the photo above. That is, I put two pieces of scrap something-or-another (1/4" wood strips in my case) at either end. Then, let the 1x2 strip naturally sag in the center, only worrying about aligning that portion with the rail base. Make sure, though, that the whole 1"x2" lines up with the base of the rail, though, in the front-to-back direction, otherwise it will be hard to glue the remainder in place. I then apply glue to only the portion that makes contact. Let that sit for a while. After that has cured, it is much easier to get the rest of the strip to align with the rail base and apply glue to it. I use a fine-tipped paint brush dunked into the styrene glue, tap it at the joint area between the base styrene and the 1"x2" strip. When you do that, give the 1"x2" strip a slight push with the brush (ever so slight) to get it to snuggle up to the base of the rail.
After it has dried for a while, I remove the weights and apply some glue to those areas that couldn't be reached due to the weights being in the way. As you can see in the photos, I work on a flat piece of glass. This is because the styrene will slightly adhere itself to the surface upon which you work, so you want a surface that doesn't have too much of a roughness to it, otherwise it will be a real chore to remove the styrene. A razor blade helps. The rail will also stick to the styrene, so you will need to work it loose as well. If you give the glue enough time to set, the rail will be able to be removed without damaging the two styrene strips. If anywhere along the joint the 1"x2" isn't firmly attached, try to fix it with some more glue before going to the next step, as the entire length of the strips must be glued to each other.
The next step in the tie plates preparations is to cut them off of the long strip into their individual pieces. I set up the NWSL Chopper to a scale 8 inches (for 9-inch wide ties), and then start chopping away. This will yield a collection of tie plates that have one 1"x2" tiny strip attached to them. This will allow the tie plate to slide under the rail and accurately stay in position. From a 14-inch strip of styrene, I get enough tie plates (~96) to cover about 25" of track (both rails), using regular mainline tie-spacing. So, while this may seem tedious, once you get the hang of it and get going, it isn't too bad. I actually find it quite enjoyable work.
Once the tie plates are in position, I lock the rail down by installing the opposite 1"x2" styrene strip. So, those need to be cut next. Using the second 1"x2" styrene strip, I use the Chopper to cut a pile of those tiny pieces. Be careful, as these have a tendency to fly off. They are also statically-charged, so they will stick to just about everything.