BTPro client is not mandatory but sure adds some fancy things.
It's an essential for me; I've become too used to it
But in all seriousness, that is something that is inevitable. The server owners can decide on the maximum amount of time someone can take to download the map and join the server, and the maximum amount of time which a client can lag behind the server. Hence from that I can deduce 2 possible causes:
1) Your router is too slow to download the map. The 250kb map, though, with your 60kbps connection, will take around 5 seconds to download IF you get a constant perfect connection speed with the server. And that in itself is already much faster than the average server's time allowance. However, it definitely isn't the server's fault as many other people can join it.
2) Your computer/CPU is too slow. This rarely or never happens on BTPro afaik (because of their vehicle limits and the time constraint of each game). But on other servers where people have like 1000 trains, it is possible that your PC takes too long to process everything that the server just dumps it. Hence your PC lags out and it falls behind the server by too significant an amount, and gets kicked. And it's does happen to me sometimes even when loading a random server during server download.
Hope you can find a solution and get playing