Final answer:
The key requirement is to include at least one steam engine and one cargo box in the train design for the thermal energy engineering lab.
Step-by-step explanation:
A key requirement for the thermal energy engineering lab regarding train design is to include at least one steam engine and one cargo box in your train design. This is a fundamental design criteria intended to simulate a real-world application of thermochemistry and heat transfer principles, which are essential in the field of Engineering Thermodynamics. It is important to consider the limitations and criteria set forth for the lab to ensure the successful completion of the project and adherence to the provided instructions, which include designing trains to handle specific track challenges like hills and gaps.