Can normal/conventional trains go underground?

Yes, full size main line trains have operated underground for many years. Historically, in 1904 the Great Northern and City Railway which connected Finsbury Park with Moorgate was opened. The bore of the tunnels was approximately 16ft.

Across the Atlantic at about the same time Penn Station NYC was opened. The only way in which main line Pennsylvania Railroad and Long Island Railroad trains can access Penn Station is via the Hudson River or East River Tunnels. Both the UK and US examples used electric traction.

Some of the answers are completely wrong, some of them are from people who have obviously never seen a "Tube" train, and others don't actually address the question you are asking.

The simple answer to your question is that Tube trains are physically smaller then main-line (conventional) trains, becasue the Tube tunnels aren't big enough for conventional trains to fit.

The business about steam engines is not really significant because although some lines of the London Underground did use steam in its early days they converted to electricity as soon as possible. Diesel power was never a serious contender for the London Underground on any line.

The London Underground has two types of tunnel: the deep level "Tube", and the sub-surface (shallow depth) tunnels. All of the trains run on standard gauge track (1.435 metres, the same as main lines) and all of them are powered by electricity supplied through the external and central rails (if you look you'll notice that all lines have 4 rails per track).

The "Tube" is a small bore tunnel (originally 12 feet in diameter) and the trains that run in them are very small and cramped. This is the traditional London Underground train that most people think of. Most of the network uses these small "loading gauge" trains even though much it is above ground.

They were built this small because the London Underground was the first in the world and at the time they didn't have the technology (or the money) to build "main line" size tunnels deep underground under the city of London without risking damaging the buildings above. This was a real risk because of the nature of the "London clay" which the city is built on. Modern tunnelling techniques now allow large-bore tunnels to be built (eg the "Crossrail" scheme).

The sub-surface lines (for example: Metropolitan, District, Circle) use trains which are closer to the dimensions of ordinary main line trains. These lines were not built under the large buildings like the Tube lines. Most were built by digging a big trench and putting a roof on to form the road above. This is why they could be built to much larger dimensions.

In some places the Tube trains and the sub-surface trains share routes, but only when on the surface. The larger trains cannot fit inside the Tube tunnels.

A few types main line trains will fit into the sub-surface tunnels but most cannot be fitted in safely because of bits that stick out around the bogies and the risk of touching the centre rail. Until the late 1970s main line trains crossed part of the Metropolitan Line at Paddington but this was stopped when a very large British Rail "Western" class diesel-hydraulic locomotive fouled a "live" rail and some very nasty (and quite noisy) things happened.

Provided the train isn't too large for the tunnel, there is no reason why "normal" trains can't go underground. The Thameslink trains from Brighton to Bedford run underground through part of London, the Heathrow Connect and Heathrow Express services run underground from near Hayes to Heathrow Airport, and tunnels are currently being bored and constructed for the Crossrail link across London from East to West. Plus of course the Eurostar trains run "underground" through the Channel Tunnel.

There are also many smaller tunnels on other mainline services.

For modern services, electric trains mean little ventilation is needed in new tunnels, but older tunnels designed for steam or diesel powered trains required many ventilation shafts if they were to run any distance.

The oldest part of the current London Underground system, the Metropolitan line into Baker Street, originally used normal (for the time, 150 years ago) steam trains. The deeper lines, like the Piccadilly and Northern lines use smaller tunnels, so require smaller trains (the ones where the doors turn in at the top towards the roof), so would be unsuitable for any normal-sized train.

The current electrical systems used to power the London Underground is different from that used elsewhere on the British railway network.

Apart from the size (to fit the tunnels) and the electrical power connections, any engineering differences between the underground trains and "normal" trains will be to allow them to cope with the tight turns in some of the tunnels, and to cope with the more frequent stopping and starting due to the short distances between stations in central London.

Technically, every conventional train can go underground if the structure gauge of the train is not bigger then that of the tunnel. but as already been stated, diesel trains are rarely used as you'd risk to intoxicate the passengers.

London Underground trains only run by third and fourth rail. They cannot use an overhead line, nor do they run on petrol.

Yes, conventional trains often go underground through tunnels, even when they were steam hauled. The "Thameslink" service from Bedford to Brighton, passes underground from St Pancras to Blackfriars, and some trains share the line from Kings Cross to Moorgate with the Underground trains.

Note Note Most of the London underground network is too small for regular trains

Yes and no.

Underground trains are electric so they do not risk the fumes poisoning people. In theory any electric train can be used underground. The channel tunnel uses electric trains.

Surface trains are made to fit the surface railway.

Underground trains are usually lower and trains from one line will not always fit another line.

The underground has a number of differing sizes of train to fit the tunnels. If it was built now this would be different but it was build by a number of companies originally.

Of course they can. Evert time a train enters a tunnel, it is usually to go underground, to pass under some obstacle such as a hill. Also, most large cities (like NYC) route trains underground to centrally located train stations.

Okay, nobody here seems to know a thing about trains. Any modern train can go underground, for a ways. I say modern, a steam train wouldn't work very well, but most modern trains would. You see, no matter the way it's generated, all modern trains are actually electric. So called diesel trains have the wheels powered by electric motors. Ever wonder how they could get control of all the engines from one engine, and get them to all pull the same way even if some were pointing backwards? It's simple when they're electric. Try doing that with other power sources. That's one reason steam engines got so large, they had a hard time working them together. Sometimes one would pull while another pushed, but that was about the extent of it. The only thing that changes is how that electricity is produced. Diesel trains have their own generators on board, powered by the diesel engines. They also have batteries, so even if there was no ventilation, they could go underground for relatively short distances off battery power. With the newer tunnels with designed-in ventilation systems, they can go as far as the tunnel goes. Electric trains powered by overhead lines would probably be less problem, as there wouldn't be a need for as much ventilation, and the diesel soot wouldn't collect on things, but that's not part of the question.

If you disregard the differences in electrical systems, main line trains would be able to run on part of the Underground as the tunnels on the District, Metropolitan, Hammersmith & City and Circle Lines are large enough.

They would not be able to run in the deep-level tube tunnels as they would be too big to fit. Trains on these lines are built to a smaller loading gauge to enable them to run through tunnels 12 feet in diameter.

Not much these days. The Tunnels are designed with ventilation systems so there is nothing special needed on the train. The old steam trains with plenty of smoke would be very messy but would still work. Electric and diesel powered trains should have no problem.