Long question: comma before a participle phrase?

Hi. Let us consider the following sentence. I think the sentence will not make sense even with a Xed-out word in terms of what it could be referring to.

XXX is a specially made vehicle with long and hard rails, often used on farms.

I think the part underlined is a past particple phrase and the word "often" is an adverb.

I also think the part "with long and hard rails" is a prepositional phrase. If we delete the what I think is a prepositional phrase, we get this.

XXX is a specially made vehicle, often used on farms.

I think whether we put a comma before the word "often" is dependent upon whether we think the part underlined is providing extra information or think it is essential to the main clause.

If we think the part underlined is essential to the main clause, then I think a comma isn't needed and thus we get this sentence.

XXX is a specially made vehicle often used on farms.

Now, if we put back the part deleted initially (after the word "vehicle"), we get this.

XXX is a specially made vehicle with long and hard rails often used on farms.

Now, to the questions I need an answer to: Could the reason for placing a comma before the part underlined be correct if it were that we could think the part underlined is modifying (?) the word "rails" instead of what it should modify (?), which is the word "vehicle"? Could we put a comma like the following before the underlined past participle phrase if we thought the prepositional phrase preceding is long and/or people could mixed up as to which is a true referent? Let us retain the assumption that the underlined past participle phrase is essential to the main clause.

XXX is a specially made vehicle with long and hard rails, (a comma here correct?) often used on farms.

Thank you for your anticipated help.