Notice that nobody is saying that it must be a short sentence or a pretty sentence. The good effects of it get overshadowed by the harm it may cause.
If you can't express the main point of your essay in one sentence, your essay probably doesn't have one point; it probably has two.
Usually, this is wishful thinking.Try to think of interesting issues, several of them. A good thesis statement will tell you when you have finished. Having to develop a written thesis statement along with your essay also helps you to discover problems with your essay and solve them. A quick test is to look at your trial thesis statement and see if it makes sense to ask either "why? This brings us to the second question. Thesis: The US should implement a cross-country high-speed rail system. To get started, use whatever techniques seem to work for you: freewriting, clustering, talking it over with friends, brainstorming. An example would be the verb "throw" in the sentence "Jane throws the ball. But it must be one sentence, not two or more sentences.
People's reactions to the scale model may help you to decide how to alter the design. I don't mean that your statement must be "positive" in the sense of optimistic, just that it must be worded as a positive claim, rather than one that uses terms like "not.
But if you can't say for sure what they add up to, what point they make, you probably don't have an essay yet.
Even if you have a clear idea of what you think you want to say before you start to write, you will usually discover that in the process of writing your idea changes.
But that is what I will ask you to do for every essay you write.