Newbie - question about steam engines

My apologies if I am in the wrong place. I have a question about steam engines that has been driving me and my mechanically inclined friends nuts. For all of our research, we can't find an explanation as to how a steam engine overcomes its at rest state and starts moving. I know about the fundamentals of the design but understanding that initial push that creates the first drive wheel rotation escapes me. When one looks at very simple farm engines, they are started by the operator spinning the flywheel by hand and obviously that can't be the case with a train engine. How is it done? If I have come to the wrong place I am sorry to take your time. If so, and you can direct me accordingly, I'd be very grateful.

Thanks

Steam under pressure from the boiler, typically between 160 and 250 psi, pushes a piston in the
locomotive cylinders connected by main rods to the crank pin on the main drive wheels, causing
the wheels to turn. Not really all that different in principal than hot expanding gas pushing a piston
connected to a connecting rod and crank in internal combustion engines except for the source of the
hot expanding gas. Check out this book: https://kalmbachhobbystore.com/product/book/01317

As to starting... If you had one cylinder, connected by a rod to one wheel, and it was stopped "dead centre" (so the rod points straight at the centre of the wheel), you'd get zero net force to rotate the wheel. Which would be a problem, since "hand-rotating the flywheel" would involve moving the train!
So the crank pins on the driving wheels on the two sides of the locomotive are "quartered": located 90 degrees apart, so if the rod on one side is dead center, the rod on the other is positioned to deliver maximum force. (This is also why typical* American* steam locomotives have a four-beat, "I think I can, I think I can," sound to their exhaust and not a two-beat "I think! I think! I think!" sound.)
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* A small number of American steam locomotives had three or four cylinders: Mallet-style articulated locomotives being the bet known. And railways in other countries -- particularly Britain and France -- had more three and four cylinder steam locomotives. ... I don't know what they sound like, but the problem of having rods positioned so as to make it hard to start would be further lessened by having multiple cylinders.
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(And, b.t.w., your question is a good one: when the star locomotive was first being invented, it required a lot of thought and ingenuity, even if the final solution was fairly simple.)

Found this video which is an animation of a Union Pacific Big Boy showing how all the parts and pieces
operate with an good narration along the way: https://www.youtube.com/watch?v=Hszu80NJ438