What is this thing?

At least in the U.S., pink paint on the ground or features, pink flagging, or pink pin flags are mostly used by surveyors. Pink is used for its high visibility to mark the ground and it's likely this paint isn't meant to last long.This could be marking project limits of a railway survey, upcoming maintenance, or a construction project. There could be other ties painted, but I wouldn't be surprised if they were miles away. It could be an indication of nearby ground control. Does "6 & 7th / PDE & 3rd" have any meaning locally? Could PDE be a street name abbreviation, considering that the other parts sound like street names? Railways are long and if it's a limit of one project or the dividing line between two projects, some of those streets might be far away.

It appears to have been painted after it was placed and after the track was laid. If you zoom you can see the paint does not run under the rails. Also there is paint on the rail clip that ties the rail to the sleeper. The bolted connection is called a fish plate I think.

It's the starting block for the train relay races.

Lmao at that completely ineffective ladder lock...

One side's harder to climb I guess.

It’s marking the joint between two track segments. Look just above it. On both tracks there’s a series of bolts indicating that two track segments have been bolted together.

Not sure why the other joints aren’t marked.

probably a change of measurement system? Look at the mileposts each side to see if there is a discontinuity in the numbers.

Breast Cancer Awareness Month

To promote awareness and inclusiveness.

Just a stab in the dark but they may occur at regular intervals, and similar notation may be on the poles and other infrastructure. In the event of a rail emergency you give the operator that info and they will halt all trains that could be affected by the emergency