Assuming you are not trolling, I am afraid your post reveals an incredibly naive understanding (or non-understanding) of what a manufacturing business needs to be successful.
To make even a 'simple' day-to-day item on a commercial basis - think plastic cutlery or even, say, wooden furniture - requires a vast array of skills and organisation from the actual artisan skills on the shop floor to marketing to legal advisers on health and Safety and employment law, etc etc. (Note, I am talking about a viable, commercial basis - not making a one off chair in your garage, though even that requires significant skills and know-how.)
To create a manufacturing operation capable of turning a mixture of raw materials from steel and other metals to various plastics using welds, screws, bolts, adhesives and I don't know what from the drawing board to a finished product requires millions and millions of man hours.
And that would be just to sell the stuff in your home country. Setting up a sales and marketing force capable of competing with established rivals for foreign sales would, in itself, be a hugely costly (and risky) project.
This is also a lesson in the cost of losing or destroying knowledge and assets. When diesels and electric traction were being considered versus steam, one of steam's strongest points was the relative cost of manufacturing. Steam locomotives were (I forget exactly) but probably something like 20% - 25% of the cost of a new diesel.
Look at the cost of Tornado - wasn't it around £3m? - and that using vast amounts of free volunteer time and gifted engineering components.
In other words, if you built a steam loco today on a purely commercial basis, it would cost far more than an equivalent diesel, because all (or at least many of) the skills associated with design and manufacture would have to be re-created.
As one poster upthread noted, BR and BREL inherited this vast array of knowledge and assets after the Big 4 were nationalised.
(As the 1960s song went, You don't realise what you've got till it's gone. )
As to the practicalities of selling rolling stock to foreign railways - see the post on loading gauges - although, of course, if you had the right product ready, off the shelf to BR loading gauges, it sill might sell a bit - eg as BREL did with Mk 2 carriages to Irish Railways in the 70s.