"Only" can be an adjective and conjunction as well as an adverb.
Always putting it in front can produce different meanings.  "He only drove tram 294 once", is different to "Only he drove tram 294 once".
In the situation here though,  "only" is being used as a synonym for "just". 
So, "Just Trams" or "Only Trams" would work, whereas "Only Tram" or "Just Tram" sounds awkward. Similarly for buses.
"Only bus lane" may be grammatically correct, but it is an abomination seeking the "only bus" that can use it.

Sent on the go with Vodafone

-------- Original message --------
From: "Vera Millsgleng20.mills@... [TramsDownUnder]" TramsDownUnder@...>
Date: 15/05/2018 10:45 AM (GMT+09:30)
To:TramsDownUnder@...
Subject: [TramsDownUnder] Proper English

Hello,

My understanding with the
way proper English should be spoke is that the adverb immediately precedes the noun it is referring to.

I.e., "ONLY TRAMS", not "TRAMS ONLY".

Another example: "ONLY BUS LANE", not "BUS LANE ONLY" (!)

Regards,

Glen Mills