Reminds of the older streetlights we used to have in Pittsburgh, the transformers were so tiny that it was hard to believe they were actually capable of running 400 watt mercury vapor lamps.
How does a Series Wired System Work?....is the Isolation Transformer there to provide lamp Voltage...and a Regulator Regulates a Steady Current for a Whole String?
if a Lamp Burns out won't the Regulator Compensate for that?....i think if a Ballast Burns out there will be an Open Circuit in the String...and the Rest of the Lights wont illuminate.
If a lamp goes out it's bypassed by the isolation ballast, so it won't affect the string. However I've seen cases of burnt up isolation ballasts disabling whole strings.