I've grown my own tomatoes from seed for decades now (Sungold) but this year, after sowing March 30th and getting 100% germination 8 days later, my seedlings (covered, under lights as usual) - two weeks after appearance are still at the two seed leaf stage - no true leaves showing. Something is wrong The only difference from past years is a new bag of seed starting mix. The seeds are from the same (bulk) pkt I have been sowing for the last 4 years. They are not leggy either.
Any suggestions as to how to jump-start these little things?

A new brand of seed starting mix or a new bag of the old stuff? Its amazing how much difference the seed starting mix can make on germination and growth. I thought it wouldn't matter what you started with as the seeds are living off the cotyledon for the first part of growth but it does. Maybe a weak watering of fertilizer?

A newly purchased bag of seed-starting mix but I don't buy by the brand so it's anybodies! I've never fertilized either my seedlings or even mature plants, with no regrets I might add

All other things equal, its either the seeds or the soil.

Well that leaves the soil as I've been working with this same bulk bag of Sungold seeds for 4 years now. I might re-pot before the true leaves appear if only to try and get tomatoes before December

Could your seed area be colder this year than usual?

Actually it's slightly warmer - around 20 C - possibly because the Grow-light is lower than usual, but it's also in a 20 C room daytime, lower at night.

I note that your seedlings are a bit spindly and wonder whether placing the lights closer would help. I used fluorescent lights and would cover the entire light with a big sheet of plastic to keep the heat in.
I've had Sungold in my garden and it grows quite tall. My plant had so many tomatoes that many fell to the ground.
The following year a few plants sprouted so I let them grow and they were Sungolds.
Ordinarily there's a 50-50 chance that a hybrid will grow new plants and stay true to its hybrid traits, but the ones that came up did.
The tomatoes on these plants are delicious and I've never seen a plant produce so many.
However, they can be dried and there are many recipes that use fresh cherry tomatoes to make a very delicious sauce for pasta.
I note you live farther north, and I was located in southern MA, so possibly this slight difference in heat zones may make a difference in the time when they sprout by themselves in a garden.

Funny you should comment Ruth! I was just examining them today; they look exactly the same as they did a week ago, no taller (not spindly) and the leaves are quite thick for seedlings and are now going bronze at the tips so possibly not long to live. The lights are actually closer than I usually put them when I use my light garden, and I had a dome on the seed tray until a day or so ago. I have been growing the seeds from the same packet for 4 years now so perhaps it is a problem with the age of the seed - young enough to germinate, too old to do much more

Update - seedlings no bigger than they were when I posted this question 3 weeks ago so I dug one up and found the seedling had virtually no roots
As I am successfully growing Stipa gigantea from 2020 seed, in the same seed medium, I have to assume it is the age of the tomato seeds (seed harvested 2020) that is causing this. I have 20 seeds left for next year and will try again, but in a different seed starting mix with perhaps 2 seeds in this year's mix as a 'control'.

Update from yesterday - because we know broken tomato stems will root in water I am conducting an experiment on the seedlings with no roots; I have dug them all up and repotted them right up to the leaves in a good soil mix and while I have missed this year for my own tomato plants it will be interesting if I can 'revive' these plants.