EDIT
The third and refined version of my sensing circuit can be found here:

https://www.instructables.com/id/cheap-USB-slotcar-...

The first version will only work reliably with a very long deadstrip or very slow cars.

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I am going to built a routed 4 lane slotcar track*.

Obviously you need some timekeeping/lapcounting solution for such a toy.
You can buy various sensors and interfaces but there is also some free software around for which you can built your own interfaces.
Most instructions are for printer or joystick ports. You can then use an USB adapter.
USB adapters are cheap but it is even cheaper to built your own timekeeping interface.

* I really got no time at the moment so I am occupying myself with short, related projects such as permanent car lightning with goldcaps, designing the track, locking at stunning sceneries...
controlling the slotcars via RC controle...

Step 1: What You Need

The software I am using (Lap Timer 2000 or Slotman) does support various input hardware.

There is native support for gamepad buttons 1-4 in LapTimer so I decided to use a gamepad for input.
It also counts if you press keys on your keyboard so my second choice would have been a USB keypad.
I ordered a cheap gamepad (3.71€, free shipping). I also ordered a cheap wireless gamepad so I could also have wireless counting.

You can use any input device if you map your input to something your software understands. Some software that worked for me is JoyToKey or Xprogger.

USB input device....USB-gamepad
copper or aluminium tape. You could even usw Aluminium foil, it will stick to the tape just like the copper tape)
wire
double sided tape

I also experimented with stripboard...I am not finished experimenting though.

There is also some soldering to do.

Step 2: Open the Gamepad

Open your gamepad and try to trigger your lap counter.
Now you have to solder some cables on the board.
Buttons 1-4 are on the right side.
Polarity does not matter, just make sure you don't short the buttons.

>>>>Before manipulating the board it is a good idea to tape the USB cable to the board!<<<<
I did not do that first and when it came to testing my interface I noticed I had ripped off the red and black cable.

You are done on the gamepad.

Step 3: The Track Side. First Try.

First I tried some stripboard. You can see in the pictures what I did.
The stripboard was too thick so flattened it on both sides with a file. It missed quite a view laps.

My second idea was to thin the stripboard. I held it to the beltgrinder and ended up with a 0.5mm stripboard but I had had no time to try it. The strips ran perpendicular to the slot and when I came back to my project I decided to run two thin copper tapes along each other.
I want to try this with two stripboard tracks also but have to thin one again.

Step 4: The Track Side. Second Try.

Now I cut a 3cm strip of 4mm copper tape in half. I did a little zig-zag pattern. Both halves should be very near to each other but should not touch. Thy get connected when the cars run over them.
I used double sided tape to stick the copper tape to it and used some more copper tape (there is one with conducting glue) to stick the wires to my pickups. You can solder wires to tape but you have to rough it up with some sandpaper first and use some flowagent to get the solder to stick to it.
A thinned stripboard would be better for that. I want to try that some other time.

Step 5: Some Thoughts.

You can see in the video that this works great (for 3.71€ invested and 30minutes work at least...). Sometimes it does not count when you run over the strip in full speed. On a routed track with copper tape speeds are a little lower because you can't use magnets. In my next try on the Carrera track I will use the stripboard pickups and make them some 2-3cm longer, this should work at any speed.
Some other way would be to built the counter in a curve where you have to slow down and the force is on the outer pickup.

I also taped off the other side of the track. Some voltage will get induced to the wires through the motor when the car is passing over. Maybe you should tape of 1cm on the beginning so that you do not get 14V from the track on the gamepad when the car bridges the track to the copper tape.

I also bought an infrared slot sensor the other day. Please feel free to help me if you know how to use these to trigger gampad buttons...my electronics are rudimentary but I am eager to learn.