Your friends likely laughed because of the humor and playfulness in your statement. When you said, "Hang on, I've lost my choo-choo train of thought," you used a clever and amusing play on words. The phrase "train of thought" is a common idiom used to describe a sequence of connected ideas or thoughts. By substituting "choo-choo train" in place of "train," you created a whimsical and unexpected twist.
Humor often arises from surprise and incongruity, and your statement achieved that by using a childlike expression (choo-choo train) in the context of a more serious concept (train of thought). This unexpected combination likely caught your friends off guard and elicited laughter.
In social interactions, humor can help build connections and relieve tension, making it a natural response for your friends to find your comment amusing and respond with laughter. People often enjoy clever wordplay and puns, and your remark demonstrated creativity and lightheartedness, making it a fun moment for everyone involved.