Мало того что поезд задерживался, так ещё и табло не работало.

It is an idiomatic structure meaning something like:

• not only ..., but ... as well
• as if ... weren’t enough, ...
• to make matters worse, ...

So this sentence means that one bad thing was already true, and then another annoying thing was true on top of that.

A natural English rendering would be:

• Not only was the train delayed, but the display board wasn’t working either.
• As if the train being delayed weren’t bad enough, the board wasn’t working too.

Literally, мало того is something like little of that or that is not enough. But in modern Russian, it works as a fixed expression and usually does not get translated word for word.

In this pattern:

• мало того что X, так ещё и Y

the idea is:

• X was already bad / already true
• and then Y happened too

So it is better to learn мало того что as one chunk rather than trying to translate each word separately every time.

Because мало того что functions as a fixed conjunction-like expression.

So Russian treats:

• мало того что

as one unit introducing the first clause.

That is why the comma comes later:

• Мало того что поезд задерживался, так ещё и табло не работало.

The comma separates the first clause from the second part of the construction.

Так ещё и strengthens the second part and adds the sense of on top of that or to make matters worse.

Breaking it down loosely:

• так = in this construction, it helps introduce the second clause
• ещё = still / more / in addition
• и = also / even

Together, так ещё и is very natural in this pattern and emphasizes the extra annoyance:

• ... and on top of that, ...
• ... and what’s more, ...

You will often see variations like:

• мало того что ..., так ещё ...
• мало того что ..., ещё и ...
• мало того что ..., так ещё и ...

The version with так ещё и is especially emphatic.

Задерживался is the past tense of the imperfective verb задерживаться.

Here it suggests an ongoing situation or state:

• the train was delayed
• the train was running late

If you used задержался instead, that would sound more like a completed event:

• the train got delayed / ended up delayed

In this sentence, задерживался fits well because the speaker is describing the situation they were in: the train was delayed, and the board was not working.

Yes, you might hear it, but it changes the nuance a bit.

Compare:

• поезд задерживался = the train was delayed / was being delayed
• поезд задержался = the train got delayed

In a complaint about the overall situation at the station, задерживался sounds very natural because it presents the delay as an ongoing problem.

Because табло is a neuter noun in Russian.

So in the past tense, the verb agrees with it in gender:

• masculine: работал
• feminine: работала
• neuter: работало
• plural: работали

Since табло is neuter singular, we get:

• табло не работало = the display board wasn’t working

Usually, табло is treated as an indeclinable noun. Its form stays the same in different cases.

For example:

• Я смотрю на табло.
• У табло стояли люди.
• На табло ничего не было видно.

Even though the form does not change, it is still grammatically neuter.

In this context, табло means an information display board, such as the board showing train times, platform numbers, delays, and so on.

At a station, it would often be understood as:

• the departures/arrivals board
• the electronic display board

So табло не работало means the station board was out of order or not displaying information.

Because поезд is the subject of the first clause:

• поезд задерживался = the train was delayed

In Russian, the subject of a clause is normally in the nominative case.

Likewise, in the second clause:

• табло is also the subject
• so it is also in the nominative

It is completely normal standard Russian, but the pattern мало того что ..., так ещё и ... has a strong expressive tone. It is very common when complaining, emphasizing, or piling up problems.

So it often sounds:

• emotional
• emphatic
• slightly dramatic in a natural way

It is common in both speech and writing.

Yes, sometimes. Russian allows some variation here.

You may hear:

• мало того что ..., так ещё и ...
• мало того что ..., так ещё ...
• мало того что ..., ещё и ...

The full version with так ещё и is especially strong and idiomatic. Omitting и or так may make it slightly less emphatic, depending on context.

Yes, Russian word order is flexible, but this specific construction has a fairly stable shape.

The most natural version is:

• Мало того что X, так ещё и Y.

You can sometimes move words around inside each clause for emphasis, for example:

• Мало того что задерживался поезд, так ещё и табло не работало.

But that version sounds more marked or stylistic. For learners, the safest pattern to remember is:

• Мало того что ... , так ещё и ...

Because the speaker is describing a past situation:

• поезд задерживался = the train was delayed
• табло не работало = the display board wasn’t working

Russian past tense agrees in gender and number, which is why:

• задерживался matches поезд (masculine singular)
• работало matches табло (neuter singular)

Yes. Good English equivalents are:

• Not only was the train delayed, but the display board wasn’t working either.
• The train was delayed, and to make matters worse, the display board wasn’t working.
• As if the train being delayed weren’t enough, the board wasn’t working either.

The Russian pattern often carries a slightly stronger emotional complaint than plain not only ... but also.

A very useful chunk is:

• мало того что ..., так ещё и ...

You can reuse it with many nouns and verbs:

• Мало того что было холодно, так ещё и дождь пошёл.
  Not only was it cold, but it started raining too.

• Мало того что он опоздал, так ещё и забыл документы.
  Not only was he late, but he forgot the documents too.

Learning the whole pattern as one expression will help you sound much more natural.