接近 and 接触: drawing closer and making contact
Compare model trains slowly approaching with parts that make a sound on contact, distinguishing a change in distance from a condition involving touch.
Model trains drawing closer
接近 (せっきん) means approaching or drawing closer. 「二台の模型列車がゆっくり接近した。」 says that the two model trains slowly approached each other.
The sentence reports a past change in the distance between the trains. Drawing closer alone does not establish that they actually touched.
A sound when parts touch
In this example, 接触 (せっしょく) is physical contact between parts. 「二つの部品が接触すると音が鳴る。」 says that the two parts make a sound when they come into contact.
接触すると supplies the condition for the sound. Do not turn it into a report of a past event in which the parts already touched and made a sound.
Distance changing and contact occurring
The model-train example with 接近 describes a change in distance. The parts example with 接触 describes a condition involving touch.
These examples compare the spatial relationship between two objects. They neither restrict every use of 接触 to physical touch nor make approach itself proof of contact.
Does 「二台の模型列車がゆっくり接近した。」 also establish that the trains touched?
Show the answer and why
No. It reports that they slowly drew closer, without supplying information that they actually touched.