I can't see why transport would stop in winter in Irkutsk/Baikal, but I can say the road to Listvyanka (the nearest village to Irkutsk) is along quite a hilly road...plus the town (and even Irkutsk itself) are a bit of a dump.

Novosibirsk if you like ballet and theatre, their hall is actually bigger than the Bolshoi in Moscow and much cheaper, plus the zoo there has one of the very few LIGERS in the world! Krasnoyarsk is also OK for nature and its zoo too. Both of these are gateways to some spectacular nature too.

Kazan is a nice day stop too. I second Ekaterinburg, especially post world cup. Ufa could be an option for Ural mountain escapes as well.

The Neva should be frozen but do you really want to risk walking on it?

The Trans Siberian is adventurous anyway no matter the weather. Trains are a huge form of transport for locals and aren't delayed so easily by poor weather so you will be fine, just dress appropriately. The trains should have heating on board and will have free hot water to use for tea etc.

I have done the Transmongolian,though it was some time ago.And not starting from Helsinki.

What would you like to know?

Have you read any guidebooks? The Trans Siberian Handbook is useful.

How long can you take for the whole trip?

I thought that this question was a classic troll, but, as indicated below from Wikipedia, times have changed. I took the Trans-Siberian Railway in the 1980s, and I have driven the Alaska Highway several times, and did not believe that there was an all weather road, yes driving on snow, across Siberia.

Are there filling stations every 200 or so KM with gasoline? I assume that diesel would be like tar in January. Do the filling stations have truck stop cappuccino? And heated places to sleep?

START WIKIPEDIA PASTE JOB:
The route, in places coinciding with European route E30 across a distance of about 190 kilometres (120 mi). One of its segment (Chelyabinsk-Novosibirsk) can be passed by the R402 highway via Ishim inside the Russian territory or by the R254 highway through the neighboring country of Kazakhstan. The route consists of seven federal highways:
M10 Russia Highway: St. Petersburg-Moscow, 664 kilometres (413 mi)
M5 Ural Highway: Moscow-Chelyabinsk, 1,880 kilometres (1,170 mi)
Baikal Highway:
R254: Chelyabinsk-Novosibirsk, 1,528 kilometres (949 mi) or 1,630 kilometres (1,010 mi) with R402 bypass highway inside the Russian territory.
R255: Novosibirsk-Irkutsk, 1,860 kilometres (1,160 mi)
R258 Baikal Highway: Irkutsk-Chita, 1,113 kilometres (692 mi)
R297 Amur Highway: Chita-Khabarovsk, 2,100 kilometres (1,300 mi)
A360 Lena Highway: Major branch leading to Yakutsk and northeast Siberia, 1,235 kilometres (767 mi)
R504 Kolyma Highway: Extension of A360 to Magadan, 2,031 kilometres (1,262 mi)
A370 Ussuri Highway: Khabarovsk-Vladivostok, 760 kilometres (470 mi)
Amur Highway[edit]
Main article: R297 highway (Russia)
Until 2010[2] the most problematic stretch of the highway was between Chita and Khabarovsk. The first section of this route, linking Belogorsk to Blagoveshchensk (124 km in length), was constructed by gulag inmates as early as 1949. Extended and updated between 1998 and 2001, this road forms part of the Asian route AH31 connecting Belogorsk to Dalian in China.
The Chita-Khabarovsk road remained largely unfinished up until early 2004, when Russian President Vladimir Putin symbolically opened the Amur Highway, with great swaths of forest separating major portions from one another. Jim Oliver and Dennis O'Neil rode motorbikes across Russia, along the Trans-Siberian Highway, during the last week of May and the first three weeks of June in 2004:back then, as described in Jim Oliver's book, Lucille and The XXX Road, the section between Chita and Khabarovsk was an extremely challenging undertaking among marsh, gravel, rock, mud (vulnerable to the rasputitsa seasons), sand, washboard, potholes, stream fording and detours of the elusive highway with a noticeable absence of pavement which leads into cases of probable surface tension which can cause the highway to collapse. In the following years the road, in some places was a modern paved highway with painted reflective lane-lines, while in others a single lane meandering, pockmarked, loose-gravel trail following the route of the early 20th century Amur Cart Road. Completion of a 7-metre-wide highway between Chita and Khabarovsk was slated for 2010: now the road is in very good condition, completely upgraded and enlarged and with a smooth surface.[3] The Amur Highway was fully reconstructed and paved in September 2010.

Yes, in the 1980s the highway ended, I believe around Nerchinsk, where it was necessary to put it onto a train for a stretch of a few hundred kilometres until the road picked up again in Yerofy Pavlovich.

Nowadays there is a paved road all the way, one does not drive on snow unless there is recent snowfall which is not cleared.

Waxed diesel is no good to anyone. Russian winter diesel has pour-point depressants which keep it liquid to around -40º C. Where it is colder than this trucks have insulated / heated tanks, or (as I did) people choose petrol-engined vehicles.

There are fuel stations every 200 km or so in the most remote stretches. Most sell hot drinks and snacks. Some are close to hotels / restaurants. Heated garages are usually only found in cities. When it was below -30º C and I had no heated parking, I kept the engine running 24 hours.

More northern parts of Siberia can be accessed by winter-only ice roads ('zimniks') but the notion some inexperienced people have of just freely driving on frozen rivers is incorrect. Ice roads need to be maintained and are only made in specific areas.

EO

My partner similar issue but managed no prob on Trans Siberian - you dont have to lock cabin everytime you go toilet, you'll be sharing anyway Russian trains are safe your stuff will be ok while u go toilet - unlikely you'll be sleeping naked so dont have to get fully dressed to go toilet just slip on slippers or shoes thats wat people do -- your over thinking the whole experience. --- maybe go on a short trip like Moscow to Irkutsk (3 days) would be better so you can try

I'm taking the Trans-Siberian railway in a few months and I'm trying to plot out my journey. On my final morning in Olkhon Island, I will try to get the earliest bus back that leaves around 7:30am-8am (on a Friday). That supposedly puts me back in Irkutsk around 1:30 pm.

I live in Irkutsk since birth (and even started a blog about Irkutsk) so as a local I just want you to plan 3-5 extra hours when travelling from Olkhon back to Irkutsk. Because in Summer it can be a huge line of cars waiting for the ferry. And even since the minibuses stand in short line (for islanders and public transport) it can take 2-3 hours of waiting. Way back also can take one more hours because of road police, or stops for the lunch or the toilet (which isn't taken into account by the schedule).

Speaking of Ulan Ude and Khabarovsk... Ulan Ude itself doesn't have much to offer itself (especially comparing to Irkutsk with its fancy coffees shops, boutique hotels and craft beer pubs). Since Irkutsk has a big Buryat population you won't miss anything, including the cuisine, and datsans. So my advice - stay longer in Khabarovsk - it is one of the best cities in Russia - with lots of 19th-century buildings, creative locals, and nice public spaces.

I have not found a company yet. I am trying to gather and sort through as much information as possible. Here is what I have to work with so far:

http://www.trans-siberian-travel.com/trans-siberian-journeys/train-compartments.html
https://www.trains-and-cruises.com/

https://www.onthegotours.com/Trans-Siberian-Railway/Planning-a-journey
Internet search

Are the ones I found via online or asking others. Are there other companies I can consider? Is the list of ones I currently have good or bad?

You could use one of the dozens after dozens of "visa support" companies that provide a.blanket invitation for the whole duration of your trip, charging anywhere between 15 dollars and 40 euro.

That said, you might want to reconsider doing the trans-siberian (or a part thereof) just for the sake of doing trans-siberian and find some stops the would pique your interest for a couple of days somewhere along the way.