MorganS:
You can't take the processor off the Teensy board. The chip which makes the MK20D work like an Arduino is not available unless you buy the whole Teensy.For the audio amplifier, buying that Adafruit item as a pre-built module is going to save you a gigantic amount of time and effort. If you can mount that over the top of your 4"x2" board then it will be silly to try to copy that into your main PCB.
Yea I just plan to mount the teensy to the pcb directly - the teensy is much too complicated.
wvmarle:
Sounds like a lot for that board - but it also depends on how much vertical space you have to place those components. For starters you'll have to go for SMD components where possible, otherwise it really won't work. I really doubt you need four layers, there's a lot you can do in two. I've done what sounds like at least this complex a circuit on a two-layer PCB with only a handful of traces on the back.SD card reader also takes a lot of space on your PCB, unless you take one on a separate breakout board and place it vertical. Remember to allow for space somewhere to take your SD card out.
You'll also probably have to go double sided (components on both sides) and go for the smallest packages possible (and yes you can hand solder 0.65mm pitch VSSOP packages, it's quite easy actually). As you put your Teensy on a socket you'll have a lot of free space under it. Great for all kinds of surface mount components.
I think you're correct about doing this in two layers. But If I end up using the pre-built max 9744 board I may have to do 3. if each layer is no more than half an inch (including the pcb and components) this will probably be fine in most locomotives. I'd like it to work in future locomotives as well, ideally, so that's why I'm trying to go as small as possible.
I could probably save space by knocking those audio jacks off the max9744 and directly soldering to the board rather than using the force-fit screw in wire holders (or whatever they are called).
I might be able to go with the teensy LC or 3.2 and have a separate vertically mounted SD reader. This might actually be a better idea depending on the loco.
Also, the 13A for the motor controller is an absolute max. Its likely going to be much less than that unless the loco is pulling a long and heavy train. Right now, I measured the MD10C at 100 degrees F, so it does get quite toasty even at minimal loads (about 1-2 amps).
Anyhow, thank you everyone for all of the wonderful suggestions on modules! I'll definitely check out Eagle, I keep hearing thats all the rage. I managed to secure a copy of orCad as well, so might try that too.
Apologies for the late reply, have been dealing with school. Cheers!