Light rail grade separation is certainly feasible, but for the more common lines (i.e. not built as part of a freeway, like VTA south of downtown and the LA Green Line were) it tends to focus on specific problem locations. Sacramento, for instance, has grade separations over the railroads (obviously needed), Power Inn Rd., South Watt Ave., Sunrise Blvd., and I think a couple of places in South Sac. The Power Inn and Watt separations were added well after the line was built (close to 20 years). The northern line is naturally grade separated for probably half its length because it's alongside UP (SP) or on a failed freeway so the main streets that bridge over also cross the LR line; unfortunately but necessarily, most of the rest of the line runs in the street.

This follows from the key point of light rail: cheap (relatively, as rail transit goes). The line is usually built with grade crossings, sometimes substantial amounts of single track, on (if available) old (or shared) railroad r/w, all to keep costs down. LR isn't always successful in reaching "cheap" (ummm ... the SF Central Subway tunnel at about $1B/mile?) but it's usually way less expensive than BART or even generic NY-style subway/elevated.

Admittedly, there has been a tendency in newer lines to do more separations up front. San Diego's Mission Valley line as an example. Those are less cheap, but were considered worth doing at the time. LR is just more flexible that way than heavy rail.

I think there are some spot separations on the LA Blue Line, and of course it enters downtown in a short subway (shades of PE, but on the other side of downtown). Here's one at Slauson, for instance ( [www.google.com] ), though it's probably an accident of having to get over the railroad a little way south. Here's one at Firestone that seems to be less of an accident ( [www.google.com] ), though maybe there was already something there for the railroad so Blue Line followed suit. Aha! One that's not arguably related to a railroad: Rosecrans ( [www.google.com] ). On the whole, though, you're right that the Blue Line seems to be light on the grade separations considering the traffic on some of the cross and occasionally parallel streets, and the service frequency and ridership (far greater than most LR lines). But remember when it was built - it was one of the first modern new-construction LR lines, and CHEAP was a major (if not primary) design criterion at the time.