i have a bugaboo and it works quite well. nothing will be that easy to get on the older trams, but the thing to consider when you are possibly needing to use a pram on one is the width of the pram and the weight.
the other thing is whether you envision taking baby out of pram and picking pram up, or keeping baby in pram and lifting them up onto the tram...

i *think* that it would be easier to have a stroller for tram usage as you can then organise your bags etc and put bubs in carrier (or on hip) then pick up the stroller and carry it on as well.
of course this option can be tricky if you have heaps of shopping, or bubs is asleep.

personally i have taken a deep breath each time and used the bugaboo and have managed ok on my own on the older trams (we NEVER get the awesome flat trams ). the only thing that can be hazardous are the pneumatic tyres getting 'stuck' under the bottom step of the tram (this is when the tyres havent been pumped in ages and DD and shopping weighs a tonne). i had this happen twice in the last 18 months and for that second it can be scary!

you'll usually find that people can help you up, going down the stairs is way easier!