If the motors in the locomotives are modern and only draw a low current then a voltage step-up module (such as an MT3608) and a 1S LiPo might be the best option as it makes charging easier. Just step up the voltage by the smallest amount that is necessary to get the performance you require.
...R
If it can be programmed with the AVRisp then it should be programmable with an Uno acting as the ISP. I program the Attiny84, 45 and 1634 with the Uno as ISP. The Arduino IDE includes a program called ArduinoISP.
...R
An L298N is a very poor choice for a stepper motor driver. You should get a specialised stepper motor driver such as a Pololu DRV8825. A specialised driver has the ability to limit the current to protect the motor and it also takes a lot of the computational load off the Arduino.
HOWEVER you need to choose a driver that can comfortably supply the current required by your motor and I don't know what that is - it will be stated in the motor specifications. The DRV8825 is OK for motors that need 1.7 amps or less.
...R
Stepper Motor Basics
Simple Stepper Code
also look up the AccelStepper library
I have found that some matt paint on the rail greatly improves adhesion on inclines
I have built my own BPRC equipment using the Arduino programming system for 009 models using the Kato tram chassis - that makes it very flexible but only for people who are prepared to do their own construction and programming. I have no interest in on-board sound but I might eventually experiment with speakers under the baseboards.
...R
Generally the sensor will need to be connected to a micro-controller and a wireless device to send messages and the main Arduino will need a compatible wireless receiver.
Probably the simplest MCU/wireless combination is the ESP8266 which can transmit as WiFi or using its own ESP-NOW system. The receiving Arduino will also need to be connected to an ESP8266. ESP-NOW demo
If you want to use a regular Arduino (such as nano) with the sensor then you could use nRF24L01+ modules for the wireless communication. Simple nRF24L01+ Tutorial
...R
I already have the components I want - LDRs. I don't have the patience to construct an equivalent from parts. I had the impression from your earlier Post that you had found a supplier of a ready-made IR detector that is smaller than an LDR. Apologies if I misunderstood.
...R