HOW exactly did we get here? This question, freighted with the deepest theological significance, can be answered by a journey of discovery that starts with a mouthful of spit. Meet the Izzards (BBC1, Wednesday of last week) followed the trail of the comedian Eddie Izzard's maternal ancestry by analysing his DNA: a small sample of spittle is all that is needed.

As I understand it, each individual's DNA sequence is largely shared by everyone else's, but there are significant markers that diverge from the norm. Mapping the DNA of communities that have not moved far from home means that finding in such places a match to each marker proves that your ancestors must once have lived there, and that you will be related to those who still do.

To turn this into entertaining TV, it was thought necessary to use a celebrity as a guinea pig, and fly him across the world to interact amusingly with these long-lost kinfolk. Actually, Izzard made a sensitive job of it. The story went from his earliest identifiable relations - African tribesmen, from 192,000 years ago, who are still living a hunter-gatherer existence - via Djibouti, and the Black Sea coast to Nordic Viking forebears who brought the Izzard female line to Britain.

As so often in this kind of process, the most important journey was an emotional one. Taking tea with a pair of sisters who matched his most recent marker, he, for the first time, talked about his mother's death, and the effect that it had had on him. The programme ended with an enigma: Izzard has one unique marker, for which no one can find a match. It felt powerful, after all this uncovering of facts, to finish on a note of mystery.

Anglican clerics are only engaged on their second-choice profession: they really want to be engine-drivers. The Railway: Keeping Britain on track (BBC2, Tuesdays) may well dispel such fantasies, as it depicts the harsh reality of the everyday life of railway folk. We have seen two of its six parts so far, focusing successively on King's Cross, and Leeds. I carried away two impressions: first, the impossible pressure placed on the operatives, who deal with passengers who all seem drunk, violent, or abusive, and a system that is stretched well beyond its capacity.

Second, I noted how admirably they cope with good humour, tolerance, and philosophy. No doubt there are many railway staff who do not live up to this ideal, but the camera does not dwell on them. A further significant fact is the high proportion of "ethnic-minority" staff, the public face of our transport system now predominantly black or brown, and seemingly more Christian - whatever his or her avowed faith - for it.

The rot on the railways apparently began in 1864, the date of the first Murder on the Victorian Railway (BBC2, Thursday of last week). This was a hybrid programme, part narrated documentary, part dramatic reconstruction, and part-actors in period costume striding about the historic locations. Important themes - was railway travel, by mixing up classes long kept separate, destroying the God-given fabric of society? - were raised, but fatally undermined by a pervading miasma of historical tone-deafness.