Q: MacBook Pro retina 13-inch mid 2014 keyboard and trackpad stopped working started after el capitan was installed

Try removing the wired keyboard. Otherwise, do the following:

Try these in order testing your system after each to see if it's back to normal:

1.  a. Resetting your Mac's PRAM and NVRAM

     b. Intel-based Macs: Resetting the System Management Controller (SMC)

2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:

     Boot to the Recovery HD: Restart the computer and after the chime press and hold down the

     COMMAND and R keys until the Utilities menu screen appears.

3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.

     When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD

     entry from the the left side list.  Click on the First Aid button in the toolbar. Wait until the operation completes, then quit Disk Utility and return

     to the main menu.

4. Reinstall OS X: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

i also have the same problem with my macbook air , keyboard and trackpad works before logging in El Capitan, but after logging in it doesn't work anymore and i have to use a bluetooth keyboard and trackpad, please let me know if you find a solution x

i tried all and even formatted my drive and reinstalled the OS but still doesn't work ....

I also tried all you suggested in sequence except #3, because the first aid tap was not activated in my system. My trackpad still does not work.