Even if you think what you would say is obvious, please add. This is genuinely something I think makes sense regarding local bus routes given the longevity of light rail and how infrequently routes change, but I also suffer from confirmation bias, so I'm hoping for reasons this would be a terrible idea but obviously would prefer reasons it would be an even more amazing idea than I thought.
It depends on the type of light rail.
Here in my city the trams share some of the roads with regular traffic, which not only means they can get caught in traffic (though they have priority where possible), but it also means the rails become a real tripping hazard for cyclists (over 800 injuries since 2015 at the last count). There's been an active campaign to introduce more safety measures but the council has been reluctant to do anything about it.
The tramlines are such a well-known hazard to locals that they actually put people off from cycling, which is surely counter-productive.
Are these the indented rails? Those will throw you off your bike instantly... Cycling lanes AND tramlines can coexist, but I guess the problem here is when you want to take a turn and the rails are in the middle of the road, so you're forced to just go over them? I guess they could implement some kind of underpass for cyclists and pedestrians.
CityNerd posted a really good video on this topic a few months ago. https://youtu.be/H6UD9h3hEdk
His videos are primarily US-focused, but I believe this one applies for pretty much any country.
There is also another alternative: cable car. It's a good way to add public transportation with minimal impact on the existing road layout.
But doesn't have enough stops
It's not an either/or thing. Buses are great (if they are well funded) and light rail is also great
The problem with buses is that most of the fleets are still running on fossil fuels. Buses also produce a shocking amount of waste in used tires. 
Another problem with light rail would be mountains. Trains don't like those. On the other hand, cable-cars and cog railway exists and seem to be viable solutions. The city of Lyon even has both and since the cog railway starts on a flat terrain, it is able to switch between both.
My pet conspiracy theory is that most bus routes are a false compromise sold to voters.
Depends on the road layout; if it's a long straight road then light railway makes sense. It's less maintenance, easier to operate, can move unhindered because it doesn't get stuck in traffic (edit: provided they don't share the roads).
For spaghetti road layouts though, I don't see the benefit, but I could be wrong since I'm no expert.
I was initially conceiving this point as a venn diagram, but I guess it works like this as well.
(fuck cars/fuck bikes)
I would think adding railways to places would take a long time, cost a ton of money, and without enough population it doesn't make sense. I don't know what specific area of the world you are thinking, but most of the world is pretty empty outside of major cities, and most of them probably do have rail service.
I have old rail running right behind my house that people always want to start using again. It's in pretty rough shape so the eyes is it's easy to expensive to get it up to spec and the amount of public interest is too low that it become unfeasable.
Buses on the other hand, can get plopped down instantly wherever they will fit on existing infrastructure. They can go where the demands is. You can have a spare one on the lot. I'd think it's easier to become a bus driver than a conductor. And ultimately if you need more buses, just but another, and if you decide to scrap the program, sell the bus and you have no useless remaining infrastructure.
Overall I'd it had the choice to take a bus from A to B or rail, I'd probably choose rail I'd the pickups and drop s were the same, but again, that's also much harder to do with a train. There's room for both, but here I think trains make more sense for longer distances and buses for local.
We need a whole host of public transit options that are best suited for each circumstance of a given area. I want public transit so smooth that even if you tried getting lost you end up where you wanted to go with no clue on how it happened. (last bit is an over exaggeration since i don't know how feasible that actually is lol)
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