You might be thinking of Heron of (Roman) Alexandria, who did indeed know of steam power, but thought it just a toy of not much significance. Steam power was also used to open temple doors in Alexandria, usually also the steam powered horns or what not, kind of in a show display for the temple goers. So they knew of steam, and that you could move things with steam, but no one seems to have thought of producing a vehicle on rails powered by steam, at least as far as I know.

Now, to what seems like your second question, would steam power have displaced slavery as an institution. Even if the Romans had developed steam powered engines on a rail system, probably not. For many Romans from abroad, slavery was their entrance into Roman culture and life. They were brought to Rome, as slaves, learned the language, a trade, the culture, etc. They were then often freed. In fact the Greeks considered the Romans strange for how often they freed slaves (this should by no means be considered an attempt to gloss over the brutality of Roman slavery in particular or slavery in general, "more often" doesn't mean "all the time"). To be Roman was more about culture than genetics. Slavery was part of the culture, from the word "Familia", which meant the entire household, including the slaves, to the cultural absorption of slaves from wars (and the human resource that represented; a spoil like the loot, slaves followed the gold in the triumphs).

So, no, likely such a vehicle would have just meant more slaves and loot for ever hungry Rome.

Hi, there have been a few threads on this; you may be interested in the responses of other users

• /u/ManicMarine in Someone told me that the Roman empire was well on its way to steam power before its fall. How accurate is that?

• /u/DuxBelisarius in How close were the Romans to developing/harnessing steam power?

• /u/moonstrife in I have heard that had it not been for the abundance of Slaves the Romans would likely have developed a steam engine. Is this true?

Fire-Elemental is correct: slavery wasn't made obsolete in the American South by steam engines in the 18th and 19th centuries, and it's hard to expect it would have made a difference to the Romans.

There have been a few posts about Heron's Engine, ( some linked below) most of them saying it was only a toy, some saying that because the Romans didn't have accurate machine tools and one saying that because Heron didn't fit the thing with a governor, it would not have worked. I think these criticisms are too simple. You could call a chiming clock a toy, but it's got some pretty thoughtful mechanics in it and you can make one big enough for a clock tower. Heron's engine, if it could have worked, might have been governed by simply connecting it to a steady load- like a chain of buckets lifting water. The Antikythera Device shows that it was possible for them to do some precise metalwork without modern machine tools- on a small scale.

In fact, Charles Parsons ( considered the father of steam turbine technology) did build a model of Heron's engine and got a huge amount of energy out of it. But he fitted it with good bearings and good rotating seals, and ran it in a near vacuum ( presumably in a large cast iron box) so there was no air resistance- none of these would have been available to the Romans. He put 100 lbs/square inch steam on it, ran it at 5,000 RPM, and got 20 horsepower from it. These would also have been very, very difficult for the Romans.

Parsons knew that the thing was a reaction engine, like a rocket or a jet ( or one of his turbines). A lot of that is therefore Newtonian physics- acceleration times mass of steam going in one direction will be equaled by the reaction of acceleration times mass of the ball turning in the other. Steam doesn't have that much mass, so in order to make it turn a turbine , a lot of it has to come out of the nozzle. The faster it comes out, the higher the top speed of the turbine....and the greater the efficiency. That meant Parsons had to supply high-pressure steam into his model and the model had to be big enough to use it, in order to do much work.

So, the Romans would have had to make a Heron Engine big enough to do work and strong enough to handle that steam. That's a scale problem: the Romans had lathes and could turn wooden things and might have been able to spin or turn a bronze hollow sphere...but a small one. Making a bronze sheet-metal globe the size of a coffee mug that will hold 100 pounds/square inch steam is not too hard, but making something that's the size of a basketball is much harder: that's really really big for a treadle or pole lathe, and if you're a Roman smith and just hammering it out, it's likely that you'll end up with thin spots and hard spots, and any of those could, as the ball whirls and flexes, concentrate stress and a crack could start and mayhem would ensue. There's also a problem with having rotating bearings and seals, so the high-pressure steam doesn't just leak out everywhere. At a time when seals and gaskets were various combinations of pieces of leather, lengths of greased rope or fiber, pine rosin and tar that would have been very difficult.

Then there's the problem of the boiler- something has to make all that high-pressure steam. Now there's a problem, again, one that bedeviled 18th century steam mechanics: to generate a lot of steam from something like a pot with a lid, the pot has to be very very large, to have a lot of surface area exposed to the fire to pass heat to the water. The bigger the pot, the weaker it tends to be and, if it's filled with water, the heavier and so the harder to support without it buckling or sagging. Eventually they would conquer this by making the boiler essentially into a long pipe, but this was a real barrier for them being able to safely use high-pressure steam, generally. The classic Boulton & Watt beam engines were only running at about 15 lbs steam pressure. It is very hard to imagine the Romans solving this problem.

Because Heron's Engine is a reaction engine, and reaction engines are now very important, it gets put into many, if not all, books on turbine technology. It was a Good Idea. But it would have been impossible for the Good Idea to have been developed by the Romans, difficult really for anyone to develop it until the 19th c. Before then, any mechanic would have been bedeviled by plenty of details.