douro ga zyuutaisite ita node, basu de ha naku tikatetu ni norimasita.

が marks 道路 as the subject of 渋滞していた.

So the first part literally works like:

• 道路が渋滞していた = The roads were congested

Even though the whole sentence is really about what the speaker chose to do, Japanese first states the situation: the roads were congested, and then gives the result: so I took the subway.

渋滞 is commonly used with する:

• 渋滞する = to be congested / to have a traffic jam

So:

• 渋滞していた = was congested / were congested

This is the natural way to describe traffic congestion.

You may sometimes see noun-based expressions with だった in Japanese, but with 渋滞, 渋滞している / 渋滞していた is very common and natural when talking about roads or traffic.

していた is the past form of している.

• 渋滞している = is congested
• 渋滞していた = was congested / had been congested

Here it describes the state of the roads at the time the speaker made the decision.

It does not necessarily mean the congestion had already ended before the speaker rode the subway. It often just gives the background situation in the past.

ので means because / so, and it sounds a bit more explanatory, objective, or neutral than から.

• 道路が渋滞していたので、地下鉄に乗りました。 = Because the roads were congested, I took the subway.

If you used から, it would still be correct:

• 道路が渋滞していたから、地下鉄に乗りました。

But ので often feels slightly softer and more matter-of-fact. In a sentence like this, it fits very naturally.

ではなく means not ..., but ... or instead of ....

So:

• バスではなく地下鉄に乗りました = I took the subway, not the bus = Instead of the bus, I took the subway

Grammatically, it comes from ではない:

• バスではない = it is not the bus
• バスではなく = not the bus / instead of the bus

In this sentence, it sets up a contrast between two choices:

• バス vs. 地下鉄

That で is part of では.

With nouns, ではない is a common negative pattern:

• 学生ではない = is not a student
• バスではなく = not the bus / not by bus / rather than the bus

Here は adds contrast, which fits the meaning well:

• as for the bus, not that one
• instead of the bus

So バスではなく is a natural contrastive expression, not just a random で particle.

Because the verb 乗る uses に for the vehicle you board or ride.

• 地下鉄に乗る = to ride the subway
• バスに乗る = to ride a bus
• 電車に乗る = to get on a train

So:

• 地下鉄に乗りました = I took the subway / I rode the subway

Using を with 乗る is not standard here.

Both can involve going by bus, but they focus on slightly different things.

• バスに乗る = to ride / get on the bus
• バスで行く = to go by bus

In your sentence, 地下鉄に乗りました focuses on the chosen means of transportation as something the speaker boarded.

If the sentence were rewritten with で行く, it would sound a little different in focus:

• バスではなく、地下鉄で行きました。 = I went by subway instead of by bus.

Both are natural, but 乗る is specifically tied to boarding/riding a vehicle.

The subject is understood from context: I.

Japanese often leaves out subjects when they are obvious.

So the sentence does not need 私は:

• 道路が渋滞していたので、バスではなく地下鉄に乗りました。

The speaker is clearly the one who took the subway, so saying 私は would usually be unnecessary unless you wanted extra emphasis or contrast.

Because the whole sentence is describing a past situation and a past action.

• 渋滞していた = the roads were congested
• 乗りました = I rode / took

The congestion is the background reason, and the subway ride is the action that followed.

Japanese often matches the tense to the time frame of the story being told.

Yes, it could, depending on the nuance.

• 道 = road, street, way; more everyday and broad
• 道路 = roadway, road; a bit more formal or technical

So:

• 道が渋滞していたので…
• 道路が渋滞していたので…

Both are understandable. 道路 may sound a bit more formal or report-like, while 道 is often more casual and common in conversation.

No, it is not strictly necessary, but it is very common.

The comma helps separate:

• the reason: 道路が渋滞していたので
• the main action: バスではなく地下鉄に乗りました

So it makes the sentence easier to read, especially in writing. Japanese punctuation is often more flexible than English punctuation.

Yes, that is a very good close translation.

A more literal breakdown is:

• 道路が = the roads
• 渋滞していたので = because were congested
• バスではなく = not the bus / instead of the bus
• 地下鉄に乗りました = rode the subway

Natural English versions include:

• Because the roads were congested, I took the subway instead of the bus.
• The roads were congested, so I took the subway rather than the bus.

All of these match the Japanese well.