When Britain's cross-country railway boom of the 1830s happened, engineers took great pains to avoid tunnels on their routes as much as possible. This was not just due to cost of tunneling, and building ventilation shafts, but also objections from landowners and a genuinely fierce anti-railway lobby, which took particular target at the dangers of tunnels.

Learned scientists raised health concerns, with one doctor not letting his patients go by a particular railway route with a tunnel, as the tunnel was 600 yards long, and the "stagnant air" would "remain impregnated with poisonous gases."

A Dr Dionysius Lardner wrote in 1837 for The Times about the Glenfield Tunnel which had opened 5 years before, that it was

...about a mile in length and nearly level. It is ventilated by eight shafts and I have frequently passed through it with a locomotive engine. Even when shut up in a closed carriage, the annoyance is very great...

Another early traveler in a different tunnel attested that

...a feeling of suffocation became perceptible, increasing so fearfully that had the tunnel been twice the length, I feel confident I should hardly have got through alive.

This was essentially due to the darkness more than the smell. A report by two doctors (Davy and Rothman) noted that the combination of smell and darkness was "to many persons unpleasant" and suggested putting lamps on the carriages.

There was perhaps some exaggeration to be had, but this meant alternative smokeless solutions were tried early; a cable-hauling system was attempted in 1840 on a 3.5 mile line using 14 miles of rope with steam engines on either end doing the winding and unwinding; the rope could not handle the tear and an attempt at using a steel replacement proved to be not slack enough.

An 1840 design used a pneumatic tube, which went into service in 1846 (London & Croydon Railway) but leaking pipes led to it being closed a year later. Another attempt was made (South Devon) which also failed as rats ate through the leather seals on the pipes.

(While the concept never made it to the London Underground, the idea of a pneumatic subway was revived by Alfred Ely Beach in New York, and he posited being able to carry passengers from City Hall to Central Park in 8 minutes. He managed to finish New York's first subway section in 1870 -- just a demonstration line -- using the technology, but the plan to expand was eventually grounded by politics and technical difficulties.)

Somewhat later -- in the 1860s -- the engineer Sir John Fowler attempted to make a fireless engine, storing energy with heated bricks. It went so badly that the project was buried and later became known as "Fowler's Ghost" for the essential erasure of the locomotive from history.

The 1860s was when the Underground was becoming a reality -- the first contract was given late in 1859, it had a test run in 1862, and its full opening was 1863 -- so a solution was urgently needed. The tunneling was relatively shallow (15 feet) to avoid ventilation issues, but it was still true a typical steam engine would have too heavy of exhaust. The solution was "condensing engines" which captured some of the smoke, but they couldn't capture all of it. The London Times claimed the amount that escaped was very little, although some accounts differ. One claimed

The sensation altogether was much like the inhalation of gas prepatory to having a tooth drawn.

and that they were "coughing and sputtering like a boy with his first cigar". Another account discusses the "sulphureous odor and the darkness of the tunnels" as well as the heat during the summer. So to answer your questions, in order:

Were passengers choking on smoke in poorly ventilated tunnels?

Yes, although the smell in summer was allegedly the worst.

Were the tunnels well lit?

No. (Although not complete darkness as the earlier accounts with tunnels mention.)

Would it have been a good experience to use these early subways?

No, but people did it anyway (as one book I have puts it, "despite the smell and gloomy atmosphere"). However, it should well be noted the main thing people cared about: safety. And indeed, although longer-term lung health was not well documented, there is no incident of the sensational kind of crash that afflicted travel and terrified the Victorians. Nothing like the Abergele rail disaster which happened in 1868, the same decade the Underground launched, which saw 33 people die when a train derailed and exploded on impact due to 50 barrels of paraffin oil. The Underground was, relatively speaking, safe, and passengers could deal with smoky and dark.

...

Green, O. (2019). London's Underground: The Story of the Tube. United Kingdom: White Lion Publishing.

Pragnell, H. J. (2016). Early British Railway Tunnels: the Implications for planners, landowners and passengers between 1830 and 1870 (Doctoral dissertation, University of York).

Roess, R. P., Sansone, G. (2013). The Wheels That Drove New York: A History of the New York City Transit System. Germany: Springer Berlin Heidelberg.

Rolt, L. T. C. (2007). Victorian engineering. United Kingdom: History Press Limited.

Hi RusticBohemian (great name, btw)

Just to add to the excellent replies...

They also took steps to design the underground infrastructure to mitigate these issues as well.

First off, the early underground lines ran very shallow on account of their 'cut and cover' construction technique, where tunnels would literally be constructed by digging a hole out of the ground, then covering the top back over it. This put the lines very close to the surface, and in comparatively spacious tunnels (since it was easier to dig one big hole for both directions of track than two separate tunnels).

While this was done for more reasons than simply passenger comfort, it also gave more space for smoke that escaped the condensers to dissipate (to some extent) than you'd get on more modern lines where individual tunnels get bored much closer to the size of the rolling stock.

In addition, the cut-and-cover technique also allowed oppotunities to vent the built up soot and smoke periodically in well-ventilated areas close to the surface to further minimise discomfort. If you look at photos of the Baker Street platforms, you can see the massive arches in the roof that originally led all the way up to street level, allowing engines to safely vent outside of tunnels.

Alongside this, the railway also purchased plots of land that they then didn't re-cover, leaving sections of track exposed where engines could vent between stations that lay far appart from one another. Most famously, one of these openings on Leinster Terrace is hidden by an entire fake façade so it fits in with the rest of the houses on the terrace, which I think featured in an episode of Sherlock?

Either way, if you ever get the pleasure of riding the Circle, Hammersmith & City, District, or Metropolitan lines in London, you'll notice how often they pass through brief patches of light, especially just before they pull into a station.

That's not to say it would have been pleasant by any stretch of the imagination, but it wouldn't have been as bad a more modern tube line in a steam engine. When the first deep tube line (the northern) was built, it was fully electrified from the start.

I think it's also helpful remember that the air of London above ground wasn't that great either. This was the era where it was the shipping and industrial capital of the world, had a goddam coal-fired power station at it's heart, and experienced 'pea-souper' smog so heavy blind people could get regular employment as guides for commuters trying to get around. The underground wouldn't have been pleasant, but the situation above often wasn't much better :)

Hope this helps

Have a lovely day