[para transit fellow]

If I recall, an elevated route through downtown was a problematic option:
1) Plus 15 bridges required that the porposed elevated line to be at the "plus 30" height above street OR reconfigure the relevant Plus 15 connector bridges as "Plus 30" ( extensive building modifications
2) the turning radius of the 90 turn E/w To N/S) was difficult to achieve due to existing buildings

I do recall that the topic was beaten to death before deciding to be underground

My take: it is indeed big dollars to put it underground. On the other hand, this will be infrastructure we will use for the next 75-100 years to connect across this sprawling city - maybe we should build it for the long haul?