Electronic – Need help with circuit to sense model train direction

The easiest way is to use one 16 bit shift register (or two 8 bit shift registers connected together). That would require only 3 pins.

• Data
• Clock
• Output latch

You would need a 16 bit variable where you set or clear the bits, then you shift the data through the shift register, and then activate the output latch.

You can use two 74HCT595

We don't normally talk about a NAND gate having "state". It just responds to its inputs.

Since a NAND gate usually can respond to its inputs within a few nanoseconds, while power supply voltages typically take at least a few milliseconds to come up to valid levels during turn-on, the "state" of the NAND gates output immediately after start-up is rarely relevant.

In general during start-up there will be some time when the voltage rises to the point where the circuit is basically functional, but is not guaranteed to be functional. A NAND gate will start to respond to its inputs during this time and should be completely in sync with its inputs by the time the power supply voltage reaches the minimum voltage for guaranteed operation.

lets say a NAND gate for instance, and I send a 1 through input A, and a 1 through input B.....lets say (for some reason) the A gets there faster

This can happen, and it is called a "glitch". A typical case is when A is switching from 1 to 0 and B is switching from 0 to 1. The output to the prior inputs is 0, and the output to the final inputs is 0, but it's possible to see a 1 asserted for a brief time during the transition if the change in A arrives first.

This is why designing syncronous logic is in many ways simpler than designing combinatorial logic. In syncronous logic we can arrange the timing so that the next flip-flop downstream never has a clock edge arrive during the time when the upstream logic might glitch, so the glitch never propagates through to be seen at the final outputs of the system.